题目

题目链接：
https://www.nowcoder.com/practice/a9c170bfaf7349e3acb475d786ab1c7d

核心

DFS+双端队列

参考答案Java

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param s string字符串
     * @return int整型
     */
    public int calculate (String s) {
        //dfs,队列
        return dfs(s, 0).val;
    }

    //当前来到s的i位置。
    public Info dfs(String s, int i) {
        LinkedList<String> ll = new LinkedList<>();
        int cur = 0;
        Info info = new Info();
        while (i < s.length() && s.charAt(i) != ')') {
            char c = s.charAt(i);
            if (c == ' ') {
                i++;
                continue;
            }

            if (c >= '0' && c <= '9') {
                cur = cur * 10 + (c - '0');
                i++;
            } else if (c != '(') { //遇到运算符了
                addNum(ll, cur);
                ll.addLast(c + "");
                i++;
                cur = 0;
            } else {
                info = dfs(s, i + 1);
                i = info.index + 1;
                cur = info.val;
            }
        }

        addNum(ll, cur);
        info.val = getRes(ll);
        info.index = i;
        return info;
    }

    public int getRes(LinkedList<String> ll) {
        int res = 0;
        boolean add = true;
        String cur;
        while (!ll.isEmpty()) {
            cur = ll.pollFirst();
            if (cur.equals("+")) add = true;
            else if (cur.equals("-")) add = false;
            else {
                if (add) res += Integer.valueOf(cur);
                else res -= Integer.valueOf(cur);
            }
        }
        return res;
    }

    public void addNum(LinkedList<String> ll, int num) {
        if (!ll.isEmpty()) {
            String top = ll.pollLast();
            if (top.equals("+") || top.equals("-")) {
                ll.addLast(top);
            } else {
                int cur = Integer.valueOf(ll.pollLast());
                if (top.equals("*")) num = cur * num;
                else num = cur / num;
            }
        }
        ll.addLast(num + "");
    }

    static class Info {
        int index;
        int val;
    }

}