LeetCode 214. Shortest Palindrome

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本文介绍了如何通过在字符串后添加字符使其成为回文串，并返回最短的回文串。详细步骤包括字符串反转、构建临时字符串、使用KMP算法找到最长前缀匹配并计算所需添加的字符数。

#include <string>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;

/*
  Given a string S, you are allowed to convert it to a palindrome by adding
  characters in from of it. Find and return the shortest palindrome you
  can find by performing this transformation.

  For example:
  Given "aacecaaa", return "aaacecaaa"
  Given "abcd", return "dcbabcd"
*/

string shortestPalindrome(string s) {
  string t = s;
  reverse(t.begin(), t.end());
  string tmp = t + '#' + s;
  int n = tmp.size();
  vector<int> dp(n, 0);
  for(int i = 1; i <= tmp.size(); ++i) {   // KMP algorithm
    int j = dp[i-1];
    while(j > 0 && (tmp[i] != tmp[j])) {
      j = dp[j-1];
    }
    dp[i] = (j + (tmp[i] == tmp[j]));
  }
  return t.substr(0, s.size() - dp[tmp.size() - 1]) + s;
}

int main(void) {
  cout << shortestPalindrome("abc") << endl;
}