代码实现：

解题思路：先将数组 newInterval 插入到数组 intervals 的末尾，再转换成合并区间

/**
 * 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().
 */

// 交换
void swap(int *m, int *n) {
    int temp = *m;
    *m = *n;
    *n = temp;
}

// 快排
void sort(int (*nums)[2], int l, int r) { // 左闭右开
    if (r - l <= 2) {
        if (r - l <= 1) { // 剩余个数：<= 1
            return;
        }
        if (nums[r - 1][0] < nums[l][0]) { // 剩余个数：= 2 且后者小于前者 交换
            swap(&nums[r - 1][0], &nums[l][0]);
            swap(&nums[r - 1][1], &nums[l][1]);
        }
        return;
    }
    int x = nums[l][0];
    int y = nums[l][1];
    int i = l, j = r - 1;
    while (i < j) {
        while (i < j && nums[j][0] >= x) {
            j--;
        }
        if (i < j) {
            nums[i][0] = nums[j][0];
            nums[i][1] = nums[j][1];
            i++;
        }
        while (i < j && nums[i][0] <= x) {
            i++;
        }
        if (i < j) {
            nums[j][0] = nums[i][0];
            nums[j][1] = nums[i][1];
            j--;
        }
    }
    nums[i][0] = x, nums[i][1] = y;
    sort(nums, l, i);
    sort(nums, i + 1, r);
}

int** insert(int **intervals, int intervalsSize, int *intervalsColSize, int *newInterval, int newIntervalSize, int *returnSize, int **returnColumnSizes) {
    int ret[intervalsSize + 1][2];
    for (int i = 0; i < intervalsSize; i++) {
        ret[i][0] = intervals[i][0];
        ret[i][1] = intervals[i][1];
    }
    ret[intervalsSize][0] = newInterval[0];
    ret[intervalsSize][1] = newInterval[1];
    int retSize = intervalsSize + 1;

    // 合并区间
    *returnSize = 0;
    // 按左边界（从小到大）快排
    sort(ret, 0, retSize); // 左闭右开

    *returnColumnSizes = malloc(sizeof(int) * retSize);
    int **result = malloc(sizeof(int*) * retSize); // 记录结果
    int left = ret[0][0];
    int right = ret[0][1];
    for (int i = 1; i < retSize; i++) {
        if (right >= ret[i][0]) {
            right = right > ret[i][1] ? right : ret[i][1];
        } else {
            (*returnColumnSizes)[(*returnSize)] = 2;
            result[(*returnSize)] = malloc(sizeof(int) * 2);
            result[(*returnSize)][0] = left;
            result[(*returnSize)][1] = right;
            (*returnSize)++;
            left = ret[i][0];
            right = ret[i][1];
        }
    }
    // 记录合并的最后一个区间
    (*returnColumnSizes)[(*returnSize)] = 2;
    result[(*returnSize)] = malloc(sizeof(int) * 2);
    result[(*returnSize)][0] = left;
    result[(*returnSize)][1] = right;
    (*returnSize)++;

    return result;
}