CF1400 E

永远无法理解的题

分治

单调栈 笛卡尔树

#include<iostream>
#include<algorithm>
#include<cmath>
#include<stack>
#include<cstring>
using namespace std;
const int N = 5e3 + 10;
int n;
int a[N];
int ch[N][2];
int sz[N];//操作次数

int read(){
    char k = getchar(); int x = 0;
    while(k < '0' || k > '9') k = getchar();
    while(k >= '0' && k <= '9') x = x * 10 + k - '0' ,k = getchar();
    return x;
}

inline int div_con(int now, int x)
{
    if(now == 0)    return 0;
    int l = div_con(ch[now][0], now), r = div_con(ch[now][1], now);
    sz[now] = 1 + sz[ch[now][0]] + sz[ch[now][1]];
    return min(sz[now], l + r + a[now] - a[x]);
}

int main(){
    n = read();
    stack<int> s;while(!s.empty())    s.pop();
    for(int i = 1 ; i <= n ; i++){
        a[i] = read();
        if(s.empty() || a[s.top()] < a[i]){
            s.push(i);//推进去的是编号哦 
        }else{
            int last = 0;//最后一个大于等于它的值的编号
            while(!s.empty() && a[s.top()] >= a[i]){
                last = s.top();s.pop();
                if(!s.empty() && a[s.top()] >= a[i]){
                    ch[s.top()][1] = last;
                }
            }
            ch[i][0] = last;
            s.push(i);
        }
    }
    int root = 0;//当前序列中最小的值 
    while(!s.empty()){
        root = s.top();s.pop();
        if(!s.empty()){
            ch[s.top()][1] = root;            
        }
    }
    printf("%d
", div_con(root, 0));
    
    return 0;
}