PAT 1040 Longest Symmetric String (25)

Given a string, you are supposed to output the length of the longest symmetric sub-string. For example, given "Is PAT&TAP symmetric?", the longest symmetric sub-string is "s PAT&TAP s", hence you must output 11.

Input Specification:

Each input file contains one test case which gives a non-empty string of length no more than 1000.

Output Specification:

For each test case, simply print the maximum length in a line.

Sample Input:

Is PAT&TAP symmetric?

Sample Output:

11


题目大意：找出字符串最大对称子串的长度；
思路：通过中心扩展来找对称子串， 从一个字符串的中心向两边扩展，就记录左右两边相等的字符个数；

len1=SymString(s, i, i);    查找以单个字符为中心的对称子串

len2=SymString(s, i, i+1)    查找以两个字符为中心的对称子串

注意点： max的初始值应该为1， 而不是0； 
　　　　输入应该用getline(), 否则会应为空格的出现， 导致输入不完整

类似的题：https://www.cnblogs.com/mr-stn/p/9196718.html

#include<iostream>
#include<vector>
#include<string>
using namespace std;
int SymString(string &s, int l, int r){
  int left=l, right=r, n=s.size();
  while(left>=0 && right<n && s[left]==s[right]){
    left--;
    right++;
  }
  return right-left-1;
}
int main(){
  string s;
  getline(cin, s);
  int max=1;
  for(int i=0; i<s.size()-1; i++){
    int len1=SymString(s, i, i);
    int len2=SymString(s, i, i+1);
    int len=len1>len2 ? len1 : len2;
    if(len>max) max=len;
  }
  cout<<max;
  return 0;
}