Leetcode: 18. 4Sum

Description

Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

Note: The solution set must not contain duplicate quadruplets.

Example

For example, given array S = [1, 0, -1, 0, -2, 2], and target = 0.

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

思路

• 跟15,16题思路差不多一致
• 最主要的是考虑到重复的情况

代码

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> res;
        int len = nums.size();
        if(len < 4) return res;
        int i = 0, j = 0, s = 0, e = 0, sum = 0;
        
        sort(nums.begin(), nums.end());
        for(i = 0; i < len; ++i){
            if(i > 0 && nums[i] == nums[i - 1]) continue;
            
            for(j = i + 1; j < len; ++j){
                if(j > i + 1 && nums[j] == nums[j - 1]) continue;
                
                s = j + 1; 
                e = len - 1;
                while(s < e){
                   if(s > j + 1 && nums[s] == nums[s - 1]){
                        s++;
                        continue;
                    }
                    
                    if(e < len - 1 && nums[e] == nums[e + 1]){
                        e--;
                        continue;
                    }
                    
                    sum = nums[i] + nums[j] + nums[s] + nums[e];
                    if(sum > target) e--;
                    else if(sum < target) s++;
                    else{
                        vector<int> tmp;
                        tmp.push_back(nums[i]);
                        tmp.push_back(nums[j]);
                        tmp.push_back(nums[s]);
                        tmp.push_back(nums[e]);
                        res.push_back(tmp);
                        s++;
                    }
                }
            }
        }
        
        return res;
    }
};