【medium】15. 3sum

Given an array nums of n integers, are there elements a, b, c in nums 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.

Example:

Given array nums = [-1, 0, 1, 2, -1, -4],

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

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        sort(nums.begin(), nums.end()); //使用有序数组，处理重复
        
        vector<vector<int>> ans;
        
        int len = nums.size();
        int start, end, target;
        
        for (int i=0; i<len; i++){
            //利用有序数组，去掉重复的第三个数，在剩下的数里找two sum
            if (i>0 && nums[i] == nums[i-1])  
                continue;
            
            start = i+1;
            end = len-1;
            target = 0-nums[i];
            
            // two sum
            while (start < end){
                // start 位置起始有重复
                if (start > i+1 && nums[start-1]==nums[start]){
                    start ++;
                    continue;
                }
                if (nums[start] + nums[end] == target){
                    vector<int> a;
                    a.push_back(nums[start]);
                    a.push_back(nums[end]);
                    a.push_back(-target);
                    ans.push_back(a);
                    start ++; // wrong
                }
                else if (nums[start] + nums[end] > target)
                    end --;
                else
                    start ++;
                
            }    
        }
        return ans;
    }
};