题目：

题解：

int* findDuplicates(int* nums, int numsSize, int* returnSize) {    
    int *ans = (int *)malloc(sizeof(int) * numsSize);
    int pos = 0;
    for (int i = 0; i < numsSize; ++i) {
        int x = abs(nums[i]);
        if (nums[x - 1] > 0) {
            nums[x - 1] = -nums[x - 1];
        } else {
            ans[pos++] = x;
        }
    }
    *returnSize = pos;
    return ans;
}