18. 4Sum

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]
]

solutions:

本題意是要求給定n個整數的陣列num和一個整數目標，那麼num中是否有元素a、b、c和d，使得a+b+c+d=目標值？在給出目標和的陣列中找到所有唯一的四胞胎。

先將陣列排序，第一第二個元素迴圈遍歷，第三第四可以比較sum和target的大小，選擇向那個方向進行遍歷。

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        sort(nums.begin(), nums.end());
        vector<vector<int>> res;
        if(nums.size() < 4)
            return res;
        for(int first = 0; first < nums.size() - 3; first++) {
            if(first != 0 && nums[first] == nums[first - 1])
                continue;
            for(int second = first + 1; second < nums.size() - 2; second++) {
                if(second != first + 1 && nums[second] == nums[second - 1])
                    continue;
                int left = second + 1;
                int right = nums.size() - 1;
                while(left < right) {
                    int sum = nums[first] + nums[second] + nums[left] + nums[right];
                    if(sum < target)
                        left++;
                    else if(sum > target)
                        right--;
                    else {
                        res.push_back({nums[first], nums[second], nums[left], nums[right]});
                        left++;
                        while(left < right && nums[left] == nums[left - 1]) {
                            left++;
                        }
                    }
                }
            }
        }
        return res;
    }
};