CodeForces

Problem Description

给你n个用户，和k-1个管道转换器，分别1转换为2,3，……k；
问你至少需要多少个转换器就可以满足用户需求

Input

4 3
5 5
8 4

Output

2
1
-1

代码：

#include<iostream>using namespace std;long long n, k;//sum = 1 + 2 - 1 + 3 - 1 …… + k - 1 = 1 + 1 + 2 + 3 + …… + k - 1;int judge(long long n1){    long long b = k - n1;    long long sum = (b + k - 1) * n1 / 2 + 1;//等差数列求和 + 1    return sum >= n;//代表能满足用户}int search(){    if(!judge(k - 1)) return -1;//不满足    if(judge(0)) return 0;//满足    int i = 0, j = k - 1;//i是不满足，j是满足    while(i < j)    {        int mid = (i + j) / 2;        if(judge(mid)) j = mid;//满足 缩小需要的管道数        else i = mid;        if(i == j - 1) return j;    }}int main(){    while(cin >> n >> k) {    cout << search() << endl;    }    return 0;}