Codeforces 460C Present 二分答案

题目链接：http://codeforces.com/contest/460/problem/C

题意： 

给定一个n长的序列

每次可以给w长的区间内的数增加1

最多可以增加m次

使得 最后结果中最小的数 最大

问这个最小的数是多少

二分答案判可行。

#include <cstdio>
#include <vector>
#include <algorithm>
#include <cstring>
#include <iostream>
using namespace std;
#define inf 1000000000
#define N 100005
int a[N];
int b[N], add[N];
bool ok(int x, int n, int m, int w){
    for(int i = 1; i<= n; i++)b[i] = a[i];
    memset(add, 0, sizeof add);
    int now = 0;
    for(int i = 1; i <= n; i++){
        now += add[i];
        b[i] += now;
        if(b[i]<x){
            int go = x - b[i];
            b[i] += go;
            m -= go;
            now += go;
            add[min(N-1,i+w)] -= go;
            if(m<0)return false;
        }
    }
    for(int i = n; i ; i--)
        if(b[i]<x)return false;
    return true;
}
int n, w, m;
int main() {
    while(~scanf("%d %d %d",&n,&m,&w)){
        int minn = inf, maxx = 0;
        for(int i = 1; i <= n; i++){
            scanf("%d",&a[i]);
            minn = min(minn, a[i]);
            maxx = max(maxx, a[i]);
        }
        int ans = minn;
        int l = minn, r = maxx + m;
        while(l <= r){
            int mid = (l+r)>>1;
            if(ok(mid, n, m, w))
                ans = max(ans, mid), l = mid+1;
            else 
                r = mid-1;
        }
        printf("%d\n",ans);
    }
    return 0;
}