Leetcode_3Sum

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.

Note: The solution set must not contain duplicate triplets.

For example, given array S = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]

1、暴力求解，三层循环。

2、将3元素求和问题转化为2元素求和，类似two_sum。先排序，再求解。

//3Sum class Solution {public:    vector<vector<int>> threeSum(vector<int>& nums) {        vector<vector<int>> res;        sort(nums.begin(), nums.end());        for(int i = 0; i < nums.size(); ++i) {        if(nums[i] > 0)break;        if(i > 0 && nums[i] == nums[i-1]) continue;        int target = 0 - nums[i];        int k = i + 1, j = nums.size()-1;        while(k < j) {        if(nums[k] + nums[j] == target) {        res.push_back({nums[i], nums[k], nums[j]});        while(k < j && nums[k] == nums[k+1]) {        ++k;        }        while(k < j && nums[j] == nums[j-1]) {        --j;        }        ++k;        --j;        }        else if(nums[k] + nums[j] < target) {        ++k;        }        else --j;        }        }        return res;    }};

参考：http://www.cnblogs.com/grandyang/p/4481576.html