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  • 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;
        }
    };
    
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  • 原文地址:https://www.cnblogs.com/lengender-12/p/6822043.html
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