18. 4Sum 解题记录

题目描述：

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.

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]
]

解题思路：

用了和2Sum和3Sum一样的解题思路。

代码：

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