二分搜索

#include<cstdio>
#include<iostream>
using namespace std;


int main(void)
{
    const int n = 5;
    const int k = 3;
    int a[5] = { 2,3,3,5,6 };
    
    int lb = -1, ub = n,mid;
    while (ub - lb > 1)
    {
        mid = (ub + lb) / 2;
        if (a[mid] >= k) ub = mid;
        else lb = mid;
    }
    printf("%d", ub);
    return 0;
}