214. Shortest palindrome

I. Question

1. Question Review

　　

2. Analysis

A string is given, and a character is added to the front of the string to make it a background. After the minimum characters are added, the background is formed.

 

Ii. Answers

1. Ideas:

① To handle the problem of odd and even number of characters in the return, insert '#' between each character in string S and put each character into a list.

② Subscript indexes are defined from 1 to mid, and left = index-1 and Right = index + 1 are defined;

Compare whether the elements indicated by left and right are equal. If they are not equal, index moves backward. If they are equal, left and right move backward until left <0, that is, the index-centered string is input. Index of the largest character in the retrieval Center

③ Insert the trailing molecular string of s that is not contained in the string with the largest index obtained by ② into a stringbuilder in reverse order, insert s into this stringbuilder, and return.

 1    public String shortestPalindrome(String s) { 2          3         List<Character> list = new ArrayList<>(); 4         list.add(‘#‘); 5         for(char c: s.toCharArray()) { 6             list.add(c); 7             list.add(‘#‘); 8         } 9 10         int len = list.size();11         int mid = (len - 1) / 2;12         int i = 1;13         int max = 0;14         while(i <= mid) {15             int left = i - 1;16             int right = i + 1;17             while(left >= 0 && right <= len - 1) {18                 if(list.get(left) != list.get(right))19                     break;20                 left--;21                 right++;22             }23             if(left <= 0)24                 max = i;25             i++;26         }27         max = max * 2 + 1;28         StringBuilder sb = new StringBuilder();29         for (int j = len - 1; j >= max; j--) {30             if(list.get(j) != ‘#‘)31                 sb.append(list.get(j));32         }33         sb = sb.append(s);34         return sb.toString();35     }

 

214. Shortest palindrome