第66题 4Sum

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:

• Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
• The solution set must not contain duplicate quadruplets.

    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)

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  Array Hash Table Two Pointers

Solution in C++:

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> result;
        if(nums.size()<4) return result;
        
        sort(nums.begin(), nums.end());
        
        for(int i=0; i<nums.size()-3; i++){
            for(int j=i+1; j< nums.size()-2; j++){
                int start = j+1, end = nums.size()-1;
                while(start<end){
                    int sum = nums[i] + nums[j] + nums[start]+ nums[end];
                    if(sum<target) start++;
                    else if(sum>target) end--;
                    else{
                        vector<int> curRes(4);
                        curRes[0] = nums[i];
                        curRes[1] = nums[j];
                        curRes[2] = nums[start];
                        curRes[3] = nums[end];
                        result.push_back(curRes);
                        start++;
                        end--;
                        while(start<end&&nums[start]==nums[start-1]) start++;
                        while(start<end&&nums[end]==nums[end+1]) end--;
                    }
                }
                while(j+1<nums.size()&&nums[j+1]==nums[j]) j++;
            }
            
            while(i+1<nums.size()&&nums[i+1]==nums[i]) i++;
        }
        return result;
    }
};

Note: 往上加循环层数。。

推广到k-sum，其实归根结底都是2sum问题，可以设置一个深度，一个开始位置，一层一层往里找。只有才深度=2时，找到答案并添加进result里。path上经历的所有value，可以用一个stack保存。