3Sum

2017-10-04 16:30:13

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]
]
主要思路：先排序，然后利用双指针法，注意去重。有三处要去重。

class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        //双指针法
        vector<vector<int>> res;
        vector<int> temp;
        sort(nums.begin(),nums.end());
        //先排序，用于去重。
        for(int i=0;i<nums.size();i++){
            int front=i+1,back=nums.size()-1;
            int target=-nums[i];
            while(front<back){
                if(target<nums[front]+nums[back])  back--;
                else if(target>nums[front]+nums[back]) front++;
                else{
                    temp.push_back(nums[i]);
                    temp.push_back(nums[front]);
                    temp.push_back(nums[back]);
                    res.push_back(temp);
                    
                    while(front<back&&nums[front]==temp[1])  front++;
                    while(front<back&&nums[back]==temp[2])  back--;
                    temp.clear();
                }
            }
            while(i+1<nums.size()&&nums[i]==nums[i+1])  i++;
        }
        return res;
    }
};