快速排序
#include <iostream>
#include <algorithm>
using namespace std;
const int N = 1e5 + 10;
int n;
int arr[N];
void quick_sort(int l, int r) {
    if (l >= r) return;
    int mid = l + r >> 1;
    int val = arr[mid];
    int p1 = l - 1, p2 = r + 1;
    while (p1 < p2) {
        while (arr[++p1] < val);
        while (arr[--p2] > val);
        if (p1 < p2) swap(arr[p1], arr[p2]);
    }
    quick_sort(l, p2);
    quick_sort(p2 + 1, r);
}
int main() {
    cin >> n;
    for (int i = 0; i < n; ++i) cin >> arr[i];
    quick_sort(0, n - 1);
    for (int i = 0; i < n; ++i) cout << arr[i] << " ";
    cout << "\n";
    return 0;
}
 
归并排序
#include <iostream>
#include <algorithm>
using namespace std;
const int N = 1e5 + 10;
int n;
int arr[N], tmp[N];
void merge_sort(int l, int r) {
    if (l >= r) return;
    int mid = l + r >> 1;
    merge_sort(l, mid);
    merge_sort(mid + 1, r);
    int i = l, j = mid + 1, k = 0;
    while (i <= mid && j <= r) {
        if (arr[i] <= arr[j]) tmp[k++] = arr[i++];
        else tmp[k++] = arr[j++];
    }
    while (i <= mid) tmp[k++] = arr[i++];
    while (j <= r) tmp[k++] = arr[j++];
    for (i = l; i <= r; ++i) arr[i] = tmp[i - l];
}
int main() {
    cin >> n;
    for (int i = 0; i < n; ++i) cin >> arr[i];
    merge_sort(0, n - 1);
    
    for (int i = 0; i < n; ++i) cout << arr[i] << " ";
    cout << "\n";
    return 0;
}
 
堆排序
#include <iostream>
#include <algorithm>
using namespace std;
const int N = 1e5 + 10;
int n, k;
int heap[N], sz;
void down(int u) {
    int t = u;
    int ls = u << 1;
    int rs = u << 1 | 1;
    if (ls <= sz && heap[ls] <= heap[t]) t = ls;
    if (rs <= sz && heap[rs] <= heap[t]) t = rs;
    if (t != u) {
        swap(heap[u], heap[t]);
        down(t);
    }
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);
    cin >> n >> k;
    for (int i = 1; i <= n; ++i) cin >> heap[i];
    sz = n;
    for (int i = n >> 1; i >= 1; --i) down(i);
    while (k--) {
        int res = heap[1];
        cout << res << " ";
        swap(heap[1], heap[sz--]);
        down(1);
    }
    return 0;
}
 
n n n皇后问题
#include <iostream>
#include <algorithm>
#include <cstring>
using namespace std;
const int N = 10;
int n;
char g[N][N];
bool is_valid(int x, int y) {
    for (int i = 0; i < n; ++i) if (g[i][y] == 'Q') return false;
    for (int i = 0; i < n; ++i) if (g[x][i] == 'Q') return false;
    int nx = x, ny = y;
    while (nx >= 0 && ny >= 0) {
        if (g[nx][ny] == 'Q') return false;
        nx--, ny--;
    }
    nx = x, ny = y;
    while (nx < n && ny >= 0) {
        if (g[nx][ny] == 'Q') return false;
        nx++, ny--;
    }
    nx = x, ny = y;
    while (nx >= 0 && ny < n) {
        if (g[nx][ny] == 'Q') return false;
        nx--, ny++;
    }
    nx = x, ny = y;
    while (nx < n && ny < n) {
        if (g[nx][ny] == 'Q') return false;
        nx++, ny++;
    }
    return true;
}
void dfs(int row, int k) {
    if (k == 0) {
        for (int i = 0; i < n; ++i) {
            for (int j = 0; j < n; ++j) {
                cout << g[i][j];
            }
            cout << "\n";
        }
        cout << "\n";
        return;
    }
    // 枚举列
    for (int i = 0; i < n; ++i) {
        if (is_valid(row, i)) {
            g[row][i] = 'Q';
            dfs(row + 1, k - 1);
            g[row][i] = '.';
        }
    }
}
int main() {
    cin >> n;
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            g[i][j] = '.';
        }
    }
    dfs(0, n);
    return 0;
}
 
最大和子数组
53.最大和子数组
class Solution {
public:
    int maxSubArray(vector<int>& nums) {
        const int N = 1e5 + 10;
        int arr[N];
        int f[N];
        int n = nums.size();
        for (int i = 0; i < n; ++i) {
            arr[i + 1] = nums[i];
        }
        memset(f, -0x3f, sizeof f);
        int res = -0x3f3f3f3f;
        arr[0] = 0;
        for (int i = 1; i <= n; ++i) {
            f[i] = max(arr[i], f[i - 1] + arr[i]);
            res = max(res, f[i]);
        }
        return res;
    }
};
 
爬楼梯
class Solution {
public:
    int climbStairs(int n) {
        const int N = 46;
        int f[N] = {0};
        f[0] = 1;
        f[1] = 1;
        for (int i = 2; i <= n; ++i) f[i] = f[i - 1] + f[i - 2];
        return f[n];
    }
};
 
中心扩展法求最长回文子序列
516. 最长回文子序列
采用中心扩展法, 分别枚举所有可能的回文串的中心位置
 然后再将回文串长度的类别分为奇数和偶数, 分别统计答案
#include <string>
using namespace std;
class Solution {
public:
    int countSubstrings(string s) {
        int res = 0;
        int n = s.size();
        for (int i = 0; i < n; ++i) {
            int l = i, r = i;
            while (l >= 0 && r < n && s[l--] == s[r++]) res++;
            
            l = i, r = i + 1;
            while (l >= 0 && r < n && s[l--] == s[r++]) res++;
        }
        return res;
    }
};
 
分割回文串
131.分割回文串
先用DP预处理所有合法的子串, 然后DFS所有分割方式
#include <iostream>
#include <vector>
#include <cstring>
using namespace std;
class Solution {
public:
    const int N = 20;
    bool f[20][20];
    void dfs(string &str, vector<vector<string>> &res, vector<string> &tmp, int u) {
        if (u >= str.size()) {
            res.push_back(tmp);
            return;
        }
        for (int v = u; v < str.size(); ++v) {
            if (f[u][v]) {
                tmp.push_back(str.substr(u, v - u + 1));
                dfs(str, res, tmp, v + 1);
                tmp.pop_back();
            }
        }
    }
    vector<vector<string>> partition(string s) {
        memset(f, false, sizeof f);
        int n = s.size();
        // 预处理回文子串
        for (int i = 0; i < n; ++i) {
            f[i][i] = true;
            if (i + 1 < n && s[i] == s[i + 1]) {
                f[i][i + 1] = true;
            }
        }
        for (int len = 3; len <= n; ++len) {
            for (int i = 0; i + len - 1 < n; ++i) {
                int j = i + len - 1;
                if (s[i] == s[j] && f[i + 1][j - 1]) {
                    f[i][j] = true;
                }
            }
        }
        vector<vector<string>> res;
        vector<string> tmp;
        dfs(s, res, tmp, 0);
        return res;
    }
};
 
动态规划求最长回文子序列
516.最长回文子序列
class Solution {
public:
    int longestPalindromeSubseq(string s) {
        const int N = 1010;
        int f[N][N] = {0};
        
        int n = s.size();
        for (int i = 0; i < n; ++i) f[i][i] = 1;
        for (int len = 2; len <= n; ++len) {
            for (int i = 0; i + len - 1 < n; ++i) {
                int j = i + len - 1;
                if (s[i] == s[j]) f[i][j] = f[i + 1][j - 1] + 2;
                else f[i][j] = max(f[i + 1][j], f[i][j - 1]);
            }
        }
        return f[0][n - 1];
    }
};
 
最长回文子串
5.最长回文子串
动态规划预处理每个状态是否是合法的, 同时记录最长的回文字符串
class Solution {
public:
    string longestPalindrome(string s) {
        const int N = s.size() + 10;
        bool f[N][N];
        memset(f, false, sizeof f);
        int n = s.size();
        for (int i = 0; i < n; ++i) f[i][i] = true;
        int start = 0, sz = 1;
        for (int i = 0; i < n; ++i) {
            int j = i + 1;
            if (s[i] == s[j]) {
                f[i][j] = true;
                start = i, sz = 2;
            }
        }
        for (int len = 3; len <= n; ++len) {
            for (int i = 0; i + len - 1 < n; ++i) {
                int j = i + len - 1;
                if (s[i] == s[j] && f[i + 1][j - 1]) {
                    f[i][j] = true;
                    if (j - i + 1 > sz) {
                        sz = j - i + 1;
                        start = i;
                    }
                }
            }
        }
        string res = "";
        for (int i = start; i < start + sz; ++i) res += s[i];
        return res;
    }
};
 
单调栈
42.接雨水
栈底到栈顶的存储的柱子高度是递减的, 当新加入的柱子高度大于当前栈顶的高度的时候, 说明能够形成凹槽, 然后边弹栈边计算积水面积


class Solution {
public:
    int trap(vector<int>& height) {
        const int N = height.size() + 10;
        int stack[N], top = 0;
        int res = 0;
        for (int i = 0; i < height.size(); ++i) {
            int val = height[i];
            while (top && val > height[stack[top]]) {
                int pre = stack[top--];
                if (!top) break;
                // 计算两个柱子之间的距离
                int d = i - stack[top] - 1;
                // 减去凹槽的高度
                int h = min(height[stack[top]], height[i]) - height[pre];
                res += h * d;
            }
            stack[++top] = i;
        }
        return res;
    }
};
 
双指针算法
C - Shortest Duplicate Subarray
问题陈述
 给你一个正整数
  
     
      
       
       
         N 
        
       
      
        N 
       
      
    N 和一个长度为  
     
      
       
       
         N 
        
       
      
        N 
       
      
    N 的整数序列
 请判断  
     
      
       
       
         A 
        
       
      
        A 
       
      
    A 是否存在一个非空(连续)子数组,它有一个重复值,多次出现在  
     
      
       
       
         A 
        
       
      
        A 
       
      
    A 中。如果存在这样的子数组,求最短的子数组的长度。
维护滑动窗口, 使用 s e t set set记录是否有重复元素, 如果有重复元素缩短左侧窗口, 直到没有重复元素, 然后递增右侧窗口
#include <iostream>
#include <algorithm>
#include <cstring>
#include <unordered_set>
using namespace std;
const int N = 2e5 + 10, INF = 0x3f3f3f3f;
int n, arr[N];
unordered_set<int> s;
int main() {
	ios::sync_with_stdio(false);
	cin.tie(0), cout.tie(0);
	cin >> n;
	for (int i = 0; i < n; ++i) cin >> arr[i];
	int res = INF;
	int l = 0;
	for (int r = 0; r < n; ++r) {
		while (s.count(arr[r])) {
			res = min(res, r - l + 1);
			s.erase(arr[l++]);
		}
		s.insert(arr[r]);
	}
	if (res == INF) res = -1;
	cout << res << endl;
	return 0;
}
 
11. 盛最多水的容器
贪心策略: 定义两个指针指向两侧, 每次移动高度较小的那个指针, 这样能够围成的面积有可能变大
class Solution {
public:
	int maxArea(vector<int>& height) {
		int res = 0;
		int l = 0, r = height.size() - 1;
		while (l < r) {
			int h = min(height[l], height[r]);
			res = max(res, h * (r - l));
			height[l] < height[r] ? l++ : r--;
		}
		
		return res;
	}
};
 
LCR 179. 查找总价格为目标值的两个商品
class Solution {
public:
	vector<int> twoSum(vector<int>& price, int target) {
		int l = 0, r = price.size() - 1;
		
		vector<int> res;
		while (l < r) {
			int sum = price[l] + price[r];
			if (sum == target) {
				res.push_back(price[l]);
				res.push_back(price[r]);
				break;
			}
			else if (sum < target) l++;
			else r--;
		}
		
		return res;
	}
};
 
链表中的中间节点
class Solution {
public:
	ListNode* middleNode(ListNode* head) {
//		定义快慢指针, 快指针走到终点, 慢指针走到中间
		ListNode *u = head;
		ListNode *v = head;
		while (u != nullptr) {
			if (u->next == nullptr) break;
			u = (u->next)->next;
			v = v->next;
		}
		return v;
	}
};
 
判断链表中是否含有环
class Solution {
public:
	bool hasCycle(ListNode *head) {
		ListNode *u = head;
		ListNode *v = head;
		while (u != nullptr && v != nullptr) {
			if (u->next == nullptr) break;
			u = u->next->next;
			v = v->next;
			if (u == v) return true;
		}
		return false;
	}
};
 
寻找链表中倒数第k个位置
class Solution {
public:
	ListNode* trainingPlan(ListNode* head, int cnt) {
		ListNode *u = head;
		ListNode *v = head;
		cnt--;
		while (cnt--) u = u->next;
		while (u->next != nullptr) {
			u = u->next;
			v = v->next;
		}
		return v;
	}
};
 
392. 判断子序列
#include <iostream>
#include <algorithm>
#include <vector>
#include <cstring>
using namespace std;
class Solution {
public:
	bool isSubsequence(string s, string t) {
		int i = 0, j = 0;
		int n = s.size(), m = t.size();
		if (n == 0) return true;
		while (i < n && j < m) {
			if (s[i] == t[j]) {
				if (i == n - 1) {
					cout << i << endl;
					return true;
				}
				i++;
			}
			j++;
		}
		return false;
	}
};
 
将所有0移动到数组末尾, 同时保证剩余元素相对位置不变
class Solution {
public:
	void moveZeroes(vector<int>& nums) {
		int n = nums.size();
//		i是处理好的下一个位置, j遍历整个数组
		int i = 0, j = 0;
		while (j < n) {
			if (nums[j]) {
				swap(nums[i], nums[j]);
				i++;
			}
			j++;
		}
	}
};
 
修改 + 分割回文串
1278. 分割回文串 III
f [ i ] [ j ] f[i][j] f[i][j]代表考虑前 i i i个字符并且已经分割了 j j j个回文子串的所有方案的集合
属性: 修改的最少字符
如何进行集合划分/状态转移
 考虑第 
     
      
       
       
         j 
        
       
      
        j 
       
      
    j个回文子串的起始位置 
     
      
       
        
        
          i 
         
        
          0 
         
        
       
      
        i_0 
       
      
    i0
  
     
      
       
       
         f 
        
       
         [ 
        
       
         i 
        
       
         ] 
        
       
         [ 
        
       
         j 
        
       
         ] 
        
       
         = 
        
       
         m 
        
       
         i 
        
       
         n 
        
       
         ( 
        
       
         f 
        
       
         [ 
        
        
        
          i 
         
        
          0 
         
        
       
         ] 
        
       
         [ 
        
       
         j 
        
       
         − 
        
       
         1 
        
       
         ] 
        
       
         + 
        
       
         c 
        
       
         o 
        
       
         s 
        
       
         t 
        
       
         ( 
        
       
         S 
        
       
         , 
        
        
        
          i 
         
        
          0 
         
        
       
         + 
        
       
         1 
        
       
         , 
        
       
         i 
        
       
         ) 
        
       
         ) 
        
       
      
        f[i][j] = min(f[i_0][j - 1] + cost(S, i_0 + 1, i)) 
       
      
    f[i][j]=min(f[i0][j−1]+cost(S,i0+1,i))
 时间复杂度:  
     
      
       
       
         O 
        
       
         ( 
        
        
        
          n 
         
        
          3 
         
        
       
         k 
        
       
         ) 
        
       
      
        O(n ^ 3k) 
       
      
    O(n3k)
#include <iostream>
#include <algorithm>
#include <vector>
#include <cstring>
using namespace std;
const int N = 110;
int n;
//f[i][j]考虑前i个字符, 已经分割了j个子串的最小修改字符次数
int f[N][N];
class Solution {
public:
//	计算将l到r修改为回文串需要的最小代价
	int cost(string &s, int l, int r) {
		int res = 0;
		for (int i = l, j = r; i < j; ++i, --j) {
			if (s[i] != s[j]) res++;
		}
		return res;
	}
	int palindromePartition(string s, int k) {
		n = s.size();
		memset(f, 0x3f, sizeof f);
		f[0][0] = 0;
		for (int i = 1; i <= n; ++i) {
			for (int j = 1; j <= min(i, k); ++j) {
//				如果只分割了一个子串, 那么就是从开头到当前位置
				if (j == 1) f[i][j] = cost(s, 0, i - 1);
//				枚举最后一个回文子串的起始位置
				else {
					for (int l = j - 1; l < i; ++l) {
						f[i][j] = min(f[i][j], f[l][j - 1] + cost(s, l, i - 1));
					}
				}
			}
		}
		return f[n][k];
	}
};
 
滑动窗口
1004. 最大连续1的个数 III
class Solution {
public:
	int longestOnes(vector<int>& nums, int k) {
		int n = nums.size();
		int l = 0, r = 0;
		int res = 0;
		int cnt = 0;
		while (r < n) {
			if (nums[r] == 0) cnt++;
			while (cnt > k) {
				if (nums[l] == 0) cnt--;
				l++;
			}
			res = max(res, r - l + 1);
			r++;
		}
		return res;
	}
};
 
替换后最长重复字符
class Solution {
public:
	int characterReplacement(string s, int k) {
		int n = s.size();
		int l = 0, r = 0;
//		记录每个字符出现的次数
		int cnt[26] = {0};
		int max_cnt = 0;
		int res = 0;
		while (r < n) {
			cnt[s[r] - 'A']++;
			max_cnt = max(max_cnt, cnt[s[r] - 'A']);
			if (r - l + 1 - max_cnt > k) {
				cnt[s[l] - 'A']--;
				l++;
			}
			res = max(res, r - l + 1);
			r++;
		}
		return res;
	}
};
 
2024. 考试的最大困扰度
class Solution {
public:
	int get(char c) {
		if (c == 'F') return 0;
		return 1;
	}
	int maxConsecutiveAnswers(string answerKey, int k) {
		int n = answerKey.size();
		int l = 0, r = 0;
		int cnt[2] = {0};
		int max_cnt = 0;
		int res = 0;
		while (r < n) {
			int &val = cnt[get(answerKey[r])];
			val++;
			max_cnt = max(max_cnt, val);
			if (r - l + 1 - max_cnt > k) {
				cnt[get(answerKey[l])]--;
				l++;
			}
			res = max(res, r - l + 1);
			r++;
		}
		return res;
	}
};
 
395. 至少有 K 个重复字符的最长子串
外层枚举的是不同字符的种类
class Solution {
public:
	int longestSubstring(string s, int k) {
		int n = s.size();
		int res = 0;
		for (int i = 1; i <= 26; ++i) {
			int cnt[26] = {0};
			int l = 0, r = 0;
			int type_cnt = 0;
			int tmp = 0;
			while (r < n) {
//				当前窗口中字符类型数量小于等于i
				if (type_cnt <= i) {
					int u = s[r] - 'a';
					if (cnt[u] == 0) type_cnt++;
					cnt[u]++;
					if (cnt[u] == k) tmp++;
					r++;
				}
//				当前窗口字符数量大于i, 缩小窗口
				else {
					int u = s[l] - 'a';
					if (cnt[u] == k) tmp--;
					cnt[u]--;
					if (cnt[u] == 0) type_cnt--;
					l++;
				}
				if (type_cnt == i && tmp == i) res = max(res, r - l);
			}
		}
		return res;
	}
};
 
713. 乘积小于 K 的子数组
class Solution {
public:
	int numSubarrayProductLessThanK(vector<int>& nums, int k) {
		if (k <= 1) return 0;
		
		int n = nums.size();
		int l = 0, r = 0;
		int res = 0;
		int curr = 1;
		while (r < n) {
			curr *= nums[r];
			while (curr >= k) {
				curr /= nums[l];
				l++;
			}
//			以r结尾的子数组的数量
			res += r - l + 1;
			r++;
		}
		return res;
	}
};
 
栈
删除字符串中所有相邻的重复项
class Solution {
public:
	string removeDuplicates(string s) {
		string res = "";
		for (char c : s) {
			if (!res.empty() && res.back() == c) res.pop_back();
			else res.push_back(c);
		}
		
		return res;
	}
};
                















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