题目描述

usaco的题目果然都很好呀

算法1

rmq， 预处理

C++ 代码

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
using namespace std;

const int N = 100010;
int pre[26];
int col[N];
int pl[N], pr[N];
int n, Q;
char str[N];
int rmq[N][17];

void init()
{
    for(int i = 0; 1 << i <= n; ++ i)
        for(int j = 1; j + (1 << i) - 1 <= n; ++ j)
            if(i == 0) rmq[j][i] = col[j];
            else rmq[j][i] = min(rmq[j][i - 1], rmq[j + (1 << (i - 1))][i - 1]);
}

int get_min(int l, int r)
{
    int k = log(r - l + 1) / log(2);
    return min(rmq[l][k], rmq[r - (1 << k) + 1][k]);
}

void get_p()
{
    int cnt = 0;
    for(int i = 1; i <= n; ++ i)
    {
        int c = col[i];
        if(!pre[c]) ++ cnt;
        else if(i > pre[c] + 1 && get_min(pre[c] + 1, i - 1) < c) ++ cnt;
        pl[i] = cnt;
        pre[c] = i;
    }
    memset(pre, 0, sizeof pre);
    cnt = 0;
    for(int i = n; i; -- i)
    {
        int c = col[i];
        if(!pre[c]) ++ cnt;
        else if(i < pre[c] - 1 && get_min(i + 1, pre[c] - 1) < c) ++ cnt;
        pr[i] = cnt;
        pre[c] = i;
    }
}

int main()
{
    scanf("%d%d", &n, &Q);
    scanf("%s", str);
    for(int i = 0; str[i]; ++ i) col[i + 1] = str[i] - 'A';
    init();
    get_p();
    while(Q --)
    {
        int l, r;
        scanf("%d%d", &l, &r);
        printf("%d\n", pl[l - 1] + pr[r + 1]);
    }
    return 0;
}