【题解】Subsequence Permutation

题目描述

A string s of length n , consisting of lowercase letters of the English alphabet, is given.

You must choose some number k between 0 and n . Then, you select kcharacters of s and permute them however you want. In this process, the positions of the other n-k characters remain unchanged. You have to perform this operation exactly once.

For example, if s=\texttt{"andrea"}k=4 characters \texttt{"a_d_ea"}\texttt{"d_e_aa"}\texttt{"dneraa"}

Determine the minimum k so that it is possible to sort s alphabetically (that is, after the operation its characters appear in alphabetical order).

输入格式

The first line contains a single integer t ( 1 \le t \le 1000 ) — the number of test cases. Then t test cases follow.

The first line of each test case contains one integer n ( 1 \le n \le 40 ) — the length of the string.

The second line of each test case contains the string s . It is guaranteed that s contains only lowercase letters of the English alphabet.

输出格式

For each test case, output the minimum k that allows you to obtain a string sorted alphabetically, through the operation described above.

输入输出样例

输入 #1

4
3
lol
10
codeforces
5
aaaaa
4
dcba

输出 #1

2
6
0
4

分析

要判断将字符串进行排序需要移动几个字符，自然只需要判断排序后的字符串和原字符串的差异即可。依次扫描原字符串和排序后的字符串的每一个字符，如果不同则表示原字符串的这一个字符需要与目标字符进行对换，此时累加答案即可。

可以把字符转为数字方便判断，时间复杂度 O(n\log n)

代码

#include<bits/stdc++.h>
using namespace std;
int t,n;
char a[51];
int c[51],b[51];
int main(){
    scanf("%d",&t);
    while(t--){
        scanf("%d",&n);
        for(int i=1;i<=n;i++)cin>>a[i],c[i]=a[i]-96,b[i]=c[i];
        int ans=0;
        sort(b+1,b+1+n);
        for(int i=1;i<=n;i++)if(c[i]!=b[i])ans++;
        printf("%d\n",ans);
    }
}

最后修改：2021 年 08 月 04 日 10 : 03 AM

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