LeetCode·18. 4Sum

題目：

Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums 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:

Given array nums = [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：

3-sum基礎上外加一層循環

代碼：

class Solution {
public:
    vector<vector<int> > fourSum(vector<int>& nums, int target) {
    vector<vector<int> > vec;
    int Len = nums.size();
    if(Len == 0)return vec;
    sort(nums.begin(), nums.end());
    vector<int> v(4);
    for(int i = 0; i < Len-3; ++i){
        if(i > 0 && nums[i-1] == nums[i])continue;
        if(nums[i] + nums[i+1] + nums[i+2] + nums[i+3] > target)return vec;
        else if(nums[i] + nums[Len-3] + nums[Len-2] + nums[Len-1] < target)continue;

        v[0] = nums[i];

        for(int j = i+1; j < Len-2; ++j){
            if(j > i+1 && nums[j-1] == nums[j])continue;
            if(nums[i] + nums[j] + nums[j+1] + nums[j+2] > target)break;
            else if(nums[i] + nums[j] + nums[Len-2] + nums[Len-1] < target)continue;

            v[1] = nums[j];

            for(int L = j+1, R=Len-1; L < R;){
                if(L > j+1 && nums[L-1] == nums[L]){
                    ++L;
                    continue;
                }
                if(nums[i] + nums[j] + nums[L]+nums[R] == target){
                    v[2] = nums[L];
                    v[3] = nums[R];
                    vec.push_back(v);

                    if(nums[L] != nums[R]){
                        while(L < R && nums[++L] == nums[L-1]){};
                        while(L < R && nums[--R] == nums[R+1]){};
                    }else{
                        break;
                    }
                }else if(nums[i] + nums[j] + nums[L]+nums[R] > target){
                    --R;
                }else{
                    ++L;
                }
            }
        }
    }

    return vec;
}
};

結果：