二分查找

int binarySearch(int a[], int size, int p) {
    int L = 0;//左端点
    int R = size - 1;//右端点
    int result = -1;
    while( L <= R) {
        int C = L + (R - L)/2;//为了防止L+R过大，溢出
        if(p < a[C]) {
            R = C - 1;
        } else if(p > a[L]) {
            L = C + 1;
        }else {
            result = C;
            break;
        }
    }
    return result;
}