四数之和——力扣18

题目描述

双指针法

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target){
		int n=nums.size();
		vector<vector<int>> res;
		sort(nums.begin(), nums.end());
		for(int a=0;a<n;a++){
			if(nums[a]>target && nums[a]>=0){
				break;
			}
			if(a>=1 && nums[a]==nums[a-1]){
				continue;
			}
			for(int b=a+1;b<n;b++){
				if(nums[a]+nums[b]>target && nums[a]+nums[b]>=0){
					break;
				}
				if(b>a+1 && nums[b]==nums[b-1]){
					continue;
				}
				int c=b+1, d=n-1;
				while(c<d){
					if((long)nums[a]+nums[b]+nums[c]+nums[d]>target){
						d--;
					} else if((long)nums[a]+nums[b]+nums[c]+nums[d]<target){
						c++;
					} else {
						res.push_back({nums[a], nums[b], nums[c], nums[d]});
						while(d>c && nums[d]==nums[d-1]) d--;
						while(d>c && nums[c]==nums[c+1]) c++;
						d--;  //找到答案时，双指针同时收缩 
						c++;
					}
				}
			}
		}
		return res;
	}
};