题目：

给你一个由 n 个整数组成的数组 nums ，和一个目标值 target 。请你找出并返回满足下述全部条件且不重复的四元组 [nums[a], nums[b], nums[c], nums[d]] （若两个四元组元素一一对应，则认为两个四元组重复）：

0 <= a, b, c, d < n
a、b、c 和 d 互不相同
nums[a] + nums[b] + nums[c] + nums[d] == target
你可以按 任意顺序 返回答案 。

示例 1：

输入：nums = [1,0,-1,0,-2,2], target = 0
输出：[[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]]
示例 2：

输入：nums = [2,2,2,2,2], target = 8
输出：[[2,2,2,2]]
 

提示：

1 <= nums.length <= 200
-109 <= nums[i] <= 109
-109 <= target <= 109

结果：

解题思路：

其实就是固定一个值之后调用三数之和的方法。

但是这里一定要注意越界。

代码：

/**
 * Return an array of arrays of size *returnSize.
 * The sizes of the arrays are returned as *returnColumnSizes array.
 * Note: Both returned array and *columnSizes array must be malloced, assume caller calls free().
 */
int compare(const void *a, const void *b) {
    return *(int *)a - *(int *)b;
}

int CheckIsNue(int one, int two, int tree, int fore, int **retArr, int *returnSize) {
    if (*returnSize == 0) {
        return 1;
    }
    for (int temp = 0; temp < *returnSize; temp++) {
        if (one == retArr[temp][0] && two == retArr[temp][1] && tree == retArr[temp][2] && fore == retArr[temp][3]) {
            return 0;
        }
    }
    return 1;
}

void TreeSum(int **retArr, int *nums, int numsSize, int target, int *returnSize, int **returnColumnSizes, int index) {
    for (int mid = index + 2; mid < numsSize - 1; mid++) {
        for (int left = mid - 1, right = mid + 1; left > index && right < numsSize; ) {
            long long temp = (long long)nums[left] + nums[mid] + nums[right];
            if (temp < target) {
                right++;
            } else if (temp > target) {
                left--;
            } else {
                int isNew = CheckIsNue(nums[index], nums[left], nums[mid], nums[right], retArr, returnSize);
                if (isNew) {
                    retArr[*returnSize] = (int *)malloc(sizeof(int) * 4);
                    (*returnColumnSizes)[*returnSize] = 4;
                    retArr[*returnSize][0] = nums[index];
                    retArr[*returnSize][1] = nums[left];
                    retArr[*returnSize][2] = nums[mid];
                    retArr[*returnSize][3] = nums[right];
                    *returnSize += 1;
                }
                right++;
                left--;
            }
        }
    }
    return;
}

int** fourSum(int* nums, int numsSize, int target, int* returnSize, int** returnColumnSizes){
    qsort(nums, numsSize, sizeof(int), compare);
    int **retArr = (int **)malloc(sizeof(int *) * numsSize*numsSize);
    *returnColumnSizes = (int *)malloc(sizeof(int) * numsSize*numsSize);
    *returnSize = 0;
    if (numsSize < 3) {
        return retArr;
    }
    for (int i = 0; i < numsSize; i++) {
        TreeSum(retArr, nums, numsSize, target - nums[i], returnSize, returnColumnSizes, i);
    }
    return retArr;
}