Palindrome
Time Limit: 3000MS Memory Limit: 65536K
Total Submissions: 58492 Accepted: 20318

Description
A palindrome is a symmetrical string, that is, a string read identically from left to right as well as from right to left. You are to write a program which, given a string, determines the minimal number of characters to be inserted into the string in order to obtain a palindrome.

As an example, by inserting 2 characters, the string “Ab3bd” can be transformed into a palindrome (“dAb3bAd” or “Adb3bdA”). However, inserting fewer than 2 characters does not produce a palindrome.

Input
Your program is to read from standard input. The first line contains one integer: the length of the input string N, 3 <= N <= 5000. The second line contains one string with length N. The string is formed from uppercase letters from ‘A’ to ‘Z’, lowercase letters from ‘a’ to ‘z’ and digits from ‘0’ to ‘9’. Uppercase and lowercase letters are to be considered distinct.

Output
Your program is to write to standard output. The first line contains one integer, which is the desired minimal number.

Sample Input

5
Ab3bd

Sample Output

2

Source
IOI 2000

題意：求一個序列改造構成迴文串最小步數；

思路：計算串與其逆串類最長公共子序列； ans=len-maxcom
*因為500*5000，dp[i][j]只與dp[i-1][]有關用滾動陣列優化

程式碼：

#include<iostream>
 

#include<stdio.h>
#include<string.h>
using namespace std;
int n;
char s1[5005];
char s2[5005];
int dp[2][5005];
int main()
{
    scanf("%d",&n);
    scanf("%s",s1+1);
    for(int i=1;i<=n;i++)
    s2[i]=s1[n-i+1];
    for(int i=1;i<=n;i++)
    for(int j=1;j<=n;j++)
    {   
        if(s1[i]==s2[j]) dp[i%2][j]=dp[(i-1)%2][j-1]+1;
        else dp[i%2][j]=max(dp[(i-1)%2][j],dp[i%2][j-1]);
    }
    printf("%d\n",n-dp[n%2][n]);
}