POJ 2456 Aggressive cows

http://poj.org/problem?id=2456

Description

Farmer John has built a new long barn, with N (2 <= N <= 100,000) stalls. The stalls are located along a straight line at positions x1,...,xN (0 <= xi <= 1,000,000,000). 

His C (2 <= C <= N) cows don't like this barn layout and become aggressive towards each other once put into a stall. To prevent the cows from hurting each other, FJ want to assign the cows to the stalls, such that the minimum distance between any two of them is as large as possible. What is the largest minimum distance?

Input

* Line 1: Two space-separated integers: N and C 

* Lines 2..N+1: Line i+1 contains an integer stall location, xi

Output

* Line 1: One integer: the largest minimum distance

Sample Input

5 3
1
2
8
4
9

Sample Output

3

题解：二分

代码：

#include <iostream>
#include <stdio.h>
#include <string.h>
#include <algorithm>
using namespace std;

#define inf 0x3f3f3f3f
const int maxn = 1e5 + 10;
int N, M;
int x[maxn];

bool C(int d) {
    int last = 0;
    for(int i = 1; i < M; i ++) {
        int crt = last + 1;
        while(crt < N && x[crt] - x[last] < d) {
            crt ++;
        }
        if(crt == N) return false;
        last = crt;
    }
    return true;
}

void solve() {
    sort(x, x + N);
    int lb = 0, ub = inf;
    while(ub - lb > 1) {
        int mid = (lb + ub) / 2;
        if(C(mid)) lb = mid;
        else ub = mid;
    }
    printf("%d
", lb);
}

int main() {
    scanf("%d%d", &N, &M);
    for(int i = 0; i < N; i ++)
        scanf("%d", &x[i]);
    solve();
    return 0;
}