18. 四数之和
题目描述
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
- -10^9^ <= nums[i] <= 10^9^
- -10^9^ <= target <= 10^9^
解题方法
排序+双指针
- C 语言
c
/**
* 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 my_cmp(const void* a, const void* b) { return *(int*)a - *(int*)b; }
int** fourSum(int* nums, int numsSize, int target, int* returnSize,
int** returnColumnSizes) {
int** quadruplets = malloc(sizeof(int*) * 1001);
*returnColumnSizes = malloc(sizeof(int) * 1001);
*returnSize = 0;
if (numsSize < 4) {
return quadruplets;
}
qsort(nums, numsSize, sizeof(int), my_cmp);
for (int i = 0; i < numsSize - 3; i++) {
if (i > 0 && nums[i] == nums[i - 1]) {
continue;
}
if ((long)nums[i] + nums[i + 1] + nums[i + 2] + nums[i + 3] > target) {
break;
}
if ((long)nums[i] + nums[numsSize - 3] + nums[numsSize - 2] +
nums[numsSize - 1] < target) {
continue;
}
for (int j = i + 1; j < numsSize - 2; j++) {
if (j > i + 1 && nums[j] == nums[j - 1]) {
continue;
}
if ((long)nums[i] + nums[j] + nums[j + 1] + nums[j + 2] > target) {
break;
}
if ((long)nums[i] + nums[j] + nums[numsSize - 2] +
nums[numsSize - 1] < target) {
continue;
}
int left = j + 1, right = numsSize - 1;
while (left < right) {
long sum = (long)nums[i] + nums[j] + nums[left] + nums[right];
if (sum == target) {
int* tmp = malloc(sizeof(int) * 4);
tmp[0] = nums[i];
tmp[1] = nums[j];
tmp[2] = nums[left];
tmp[3] = nums[right];
(*returnColumnSizes)[(*returnSize)] = 4;
quadruplets[(*returnSize)++] = tmp;
while (left < right && nums[left] == nums[left + 1]) {
left++;
}
left++;
while (left < right && nums[right] == nums[right - 1]) {
right--;
}
right--;
} else if (sum < target) {
left++;
} else {
right--;
}
}
}
}
return quadruplets;
}