LeetCode 15. 3Sum

题目：

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

 

思路：

1.两个指针遍历，用bin_search查询是否存在-a-b（time exceeded）

2.遍历一次，用front和back夹逼查询  https://leetcode.com/discuss/23595/share-my-solution-around-50ms-with-explanation-and-comments

都首先用std:sort()进行排序

 

代码 思路2  C++:

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        vector<vector<int>> solve;
        if (nums.size() < 3)
            return solve;
        sort(nums.begin(), nums.end());
        
        for (int ptr_1 = 0; ptr_1 < nums.size() - 2;ptr_1++ ) {
            int target = -nums[ptr_1];
            int front = ptr_1 + 1;
            int back = nums.size() - 1;
            
            while (front < back) {
                int sum = nums[front] + nums[back];
                if (target > sum)
                    front++;
                else if (target < sum)
                    back--;
                else{
                    vector<int> ans(3,0);
                    ans[0] = nums[ptr_1];
                    ans[1] = nums[front];
                    ans[2] = nums[back];
                    solve.push_back(ans);
                    while (front < back && nums[front] == ans[1])
                        front++;
                    while (front < back && nums[back] == ans[2]) {
                        back--;
                    }
                }
            }
            while (ptr_1 + 1 < nums.size() - 2 && nums[ptr_1] == nums[ptr_1 + 1]) {
                ptr_1++;
            }
        }
        return solve;
    }
};