题目：

题解：

class Solution {
public:
    int thirdMax(vector<int> &nums) {
        int *a = nullptr, *b = nullptr, *c = nullptr;
        for (int &num : nums) {
            if (a == nullptr || num > *a) {
                c = b;
                b = a;
                a = &num;
            } else if (*a > num && (b == nullptr || num > *b)) {
                c = b;
                b = &num;
            } else if (b != nullptr && *b > num && (c == nullptr || num > *c)) {
                c = &num;
            }
        }
        return c == nullptr ? *a : *c;
    }
};