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Problem solution · Java

Maximum Product of the Length of Two Palindromic Substrings

Maximum Product of the Length of Two Palindromic Substrings: a Java solution using direct simulation. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Direct simulation
Source
walkccc LeetCode Solutions
Length
54 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

Direct simulation

For Maximum Product of the Length of Two Palindromic Substrings, the implementation follows the problem’s operations directly while maintaining only the state needed for the next decision.

  1. Translate each rule into one explicit state update.
  2. Maintain the invariant after every processed item.
  3. Return the accumulated state once all relevant input has been handled.

Code notes

  • 54 lines of Java from the credited upstream file 1960.java.
  • The implementation visibly relies on sequence storage.
  • 5 loop blocks detected.

Complexity

Count the number and nesting of passes over the input, then include the maintained containers in the memory estimate.

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 codeMaximum Product of the Length of Two Palindromic Substrings · JavaJava
Use this to learn the idea, then write your own version.
class Solution {  public long maxProduct(String s) {    final int n = s.length();    long ans = 1;    // maxLeft[i] := the maximum odd length of palindromes in s[0..i]    int[] maxLeft = manacher(s, n);    // maxRight[i] := the maximum odd length of palindromes in s[i..n - 1]    int[] maxRight = manacher(new StringBuilder(s).reverse().toString(), n);     reverse(maxRight, 0, n - 1);     for (int i = 1; i < n; ++i)      ans = Math.max(ans, (long) maxLeft[i - 1] * maxRight[i]);     return ans;  }   private int[] manacher(final String s, int n) {    int[] maxExtends = new int[n];    int[] leftToRight = new int[n];    Arrays.fill(leftToRight, 1);    int center = 0;     for (int i = 0; i < n; ++i) {      final int r = center + maxExtends[center] - 1;      final int mirrorIndex = center - (i - center);      int extend = i > r ? 1 : Math.min(maxExtends[mirrorIndex], r - i + 1);      while (i - extend >= 0 && i + extend < n && s.charAt(i - extend) == s.charAt(i + extend)) {        leftToRight[i + extend] = 2 * extend + 1;        ++extend;      }      maxExtends[i] = extend;      if (i + maxExtends[i] >= r)        center = i;    }     for (int i = 1; i < n; ++i)      leftToRight[i] = Math.max(leftToRight[i], leftToRight[i - 1]);     return leftToRight;  }   private void reverse(int[] arr, int l, int r) {    while (l < r)      swap(arr, l++, r--);  }   private void swap(int[] arr, int i, int j) {    final int temp = arr[i];    arr[i] = arr[j];    arr[j] = temp;  }}

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