LeetCode 15. 3Sum

题目描述:

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]
]

参考http://xiadong.info/2016/07/1-two-sum/Two Sum这道题, 我们可以先取得一个数n, 将-n作为target就变为了Two Sum问题, 代码如下:

class Solution {
    vector<vector<int>> ret;
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        sort(nums.begin(), nums.end());

        for(int i = 0; i < nums.size(); i++){
            if(i >= 1 && nums[i] == nums[i - 1]) continue;
            if(nums[i] > 0) break;
            twoSum(nums, -nums[i], i + 1, nums.size());
        }
        return ret;
    }
    
    void twoSum(vector<int>& nums, int target, int left, int right) {
        int l = left, r = right - 1;
        while(l < r){
            int sum = nums[l] + nums[r];
            if(sum == target){
                vector<int> t = {-target, nums[l], nums[r]};
                ret.push_back(t);
                do{r--;}while(nums[r] == nums[r + 1]);
                do{l++;}while(nums[l] == nums[l - 1]);
            }
            else if(sum > target){
                r--;
            }
            else{
                l++;
            }
        }
    }
};