LeetCode 564. Find the Closest Palindrome

原题链接在这里：https://leetcode.com/problems/find-the-closest-palindrome/

题目：

Given an integer n, find the closest integer (not including itself), which is a palindrome.

The 'closest' is defined as absolute difference minimized between two integers.

Example 1:

Input: "123"
Output: "121"

Note:

1. The input n is a positive integer represented by string, whose length will not exceed 18.
2. If there is a tie, return the smaller one as answer.

题解：

First find all the candidates, add "9999..999", "10000..0001".

Add first half, + reverse of (first half). 

e.g. 12567, its first half is 125, the candiates couldbe 12521, 12421, 12621.

Then check all the candidates and find the closest one.

Time Complexity: O(mn). m = is candidates list size. n is candiate length.

Space: O(mn).

AC Java:

class Solution {
    public String nearestPalindromic(String n) {
        if(n == null || n.length() == 0){
            return n;
        }
        
        int len = n.length();
        if(len == 1){
            int val = Integer.valueOf(n);
            return val > 0 ? "" + (val - 1) : "" + (val + 1);
        }
        
        List<String> cans = new ArrayList<>();
        
        cans.add(allNine(len - 1));
        cans.add(oneZero(len + 1));
        
        int halfLen = (len + 1) / 2;
        String sub = n.substring(0, halfLen);
        if(len % 2 == 1){
            cans.add(sub + new StringBuilder(sub).deleteCharAt(halfLen - 1).reverse().toString());
            long halfVal = Long.valueOf(sub);
            long plusOne = halfVal + 1;
            cans.add("" + plusOne + new StringBuilder("" + plusOne / 10).reverse().toString());
            long minusOne = halfVal - 1;
            if(minusOne > 0){
                cans.add( "" + minusOne + new StringBuilder("" + minusOne / 10).reverse().toString());
            }
        }else{
            cans.add(sub + new StringBuilder(sub).reverse().toString());
            long halfVal = Long.valueOf(sub);
            long plusOne = halfVal + 1;
            cans.add("" + plusOne + new StringBuilder("" + plusOne).reverse().toString());
            long minusOne = halfVal - 1;
            if(minusOne > 0){
                cans.add("" + minusOne + new StringBuilder("" + minusOne).reverse().toString());
            }
        }
        
        long diff = Long.MAX_VALUE;
        String res = "";
        long nValue = Long.valueOf(n);
        for(String can : cans){
            if(can.equals(n)){
                continue;
            }
            
            long canValue = Long.valueOf(can);
            if(Math.abs(canValue - nValue) < diff){
                diff = Math.abs(canValue - nValue);
                res = can;
            }else if(Math.abs(canValue - nValue) == diff && (res.length() == 0 || canValue < Long.valueOf(res))){
                res = can;
            }
        }
        
        return res;
    }
    
    private String allNine(int len){
        char [] charArr = new char[len];
        Arrays.fill(charArr, '9');
        return new String(charArr);
    }
    
    private String oneZero(int len){
        char [] charArr = new char[len];
        Arrays.fill(charArr, '0');
        charArr[0] = '1';
        charArr[len - 1] = '1';
        return new String(charArr);
    }
}