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Problem solution · Java

Largest Palindromic Number

Largest Palindromic Number: a Java solution using hash-based lookup. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Hash-based lookup
Source
walkccc LeetCode Solutions
Length
45 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Hash-based lookup

For Largest Palindromic Number, the implementation stores previously seen values or frequencies in a hash table for direct membership and lookup operations.

  1. Decide the key that represents the information needed later.
  2. Update its count or stored state while scanning the input.
  3. Use constant-time expected lookups to detect matches or assemble the result.

Code notes

  • 45 lines of Java from the credited upstream file 2384.java.
  • The implementation visibly relies on hash lookup, ordered lookup.
  • 4 loop blocks detected.

Complexity

Expected hash operations are constant time, but the surrounding scan and the number of stored keys determine total work and memory.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeLargest Palindromic Number · JavaJava
Use this to learn the idea, then write your own version.
class Solution {  public String largestPalindromic(String num) {    Map<Character, Integer> count = new HashMap<>();     for (final char c : num.toCharArray())      count.merge(c, 1, Integer::sum);     final String firstHalf = getFirstHalf(count);    final String mid = getMid(count);    final String ans = firstHalf + mid + reversed(firstHalf);    return ans.isEmpty() ? "0" : ans;  }   private String getFirstHalf(Map<Character, Integer> count) {    StringBuilder sb = new StringBuilder();    for (char c = '9'; c >= '0'; --c) {      final int freq = count.getOrDefault(c, 0);      sb.append(String.valueOf(c).repeat(freq / 2));    }    final int index = firstNotZeroIndex(sb);    return index == -1 ? "" : sb.substring(index);  }   private int firstNotZeroIndex(StringBuilder sb) {    for (int i = 0; i < sb.length(); ++i)      if (sb.charAt(i) != '0')        return i;    return -1;  }   private String getMid(Map<Character, Integer> count) {    StringBuilder sb = new StringBuilder();    for (char c = '9'; c >= '0'; --c) {      final int freq = count.getOrDefault(c, 0);      if (freq % 2 == 1)        return String.valueOf(c);    }    return "";  }   private String reversed(final String s) {    return new StringBuilder(s).reverse().toString();  }}

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