15. 三数之和(双指针)

题目描述

leetcode - 15：https://leetcode-cn.com/problems/3sum/

解题关键

• 双指针(把 a+b+c=0 化成 求 -a=b+c )

代码

vector<vector<int>> threeSum(vector<int>& nums) {
    vector<vector<int>> ans;
    if(nums.size()<3) return ans;
    sort(nums.begin(),nums.end());
    for(int i=0;i<nums.size();i++){
        int start = i+1;
        int end = nums.size()-1;
        if( i>0 && nums[i]==nums[i-1]){
            continue;
        }
        while(start<end){
            if( nums[start]+nums[end]>-nums[i] ){
                end--;
                continue;
            }else if( nums[start]+nums[end]<-nums[i] ){
                start++;
                continue;
            }else if(nums[start]+nums[end]==-nums[i]){
                if(start!=i+1&&nums[start]==nums[start-1]&&nums[end]==nums[end+1]){
                    start++;
                    end--;
                    continue;
                }
                vector<int> tmp;
                tmp.push_back(nums[i]);
                tmp.push_back(nums[start]);
                tmp.push_back(nums[end]);
                ans.push_back(tmp);
                end--;
                start++;
            }
        }
    }
    return ans;
}