P1873 砍树

二段性：如果高度\(h\)满足要求，那么所有的满足\(\le h\)的高度都满足，若\(h\)不满足，那么所有\(\ge h\)的都不满足。
二分答案，找满足要求的最大\(h\)
复杂度：\(O(nlog(1e9)) = O(n), n \le 1e6\)

#include<iostream>
using namespace std;

const int N = 1e6 + 10;

int h[N];
int n, m;

int check(int x){
    int sum = 0;
    for(int i = 1; i <= n; i ++){
        if(x < h[i]) sum += h[i] - x;
        if(sum >= m) return 1;
    }
    
    return 0;
}

int main(){
    cin >> n >> m;
    
    for(int i = 1; i <= n; i ++) cin >> h[i];
    
    int l = 0, r = 1e9;
    
    while(l < r){
        int mid = l + r + 1 >> 1;
        if(check(mid)) l = mid;
        else r = mid - 1;
    }
    
    cout << l << endl;
    
    return 0;
}