// 若找到，返回下标（1‑based）；否则返回 -1
int binarySearchLeft(int a[], int n, int x) {
    if (n <= 0) return -1;
    int l = 1, r = n;
    while (l < r) {
        int mid = (l + r) / 2;
        if (a[mid] >= x) r = mid;
        else l = mid + 1;
    }
    return (a[l] == x) ? l : -1;
}