15_三数之和C++

排序+双指针的方法是很容易想到的

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        sort(nums.begin(), nums.end());
        int n = nums.size();
        vector<vector<int>> ans;
        for (int first = 0; first < n;first++)//第一重循环
        {
            if (nums[first] > 0)
                break;
            if (first > 0 && nums[first] == nums[first - 1])
                continue;//判断是否开始第二重循环
            int third = n - 1;
            int target = -nums[first];
            for (int second = first + 1; second < n; second++)
            {
                if (second > first+1 && nums[second] == nums[second - 1])
                    continue;//判断是否开始第三重循环
                while (third > second && nums[second] + nums[third] > target)
                    --third;
                if (third==second)
                    break;
                if (nums[second] + nums[third] == target)
                {
                    ans.push_back({ nums[first], nums[second], nums[third] });//不用判断third重复
                }
            }
        }
        return ans;
    }
};

这样才能把时间复杂度降低到O(N^2)
O(N^3)的情况是一定会超时不通过的