二分查找

int binarySearch(int a[], int n, int target) {
    int l = 1, r = n;
    while (l <= r) {
        int mid = (l + r) / 2;
        if (a[mid] == target) return mid;       // 正中目标
        else if (a[mid] < target) l = mid + 1;  // 目标在右半
        else r = mid - 1;                       // 目标在左半
    }
    return -1;  // 没找到
}