[leetcode] 4Sum

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)

思路：

与3Sum 一样，只是多了一个数字，所以多了一层循环。

题解：

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