687. Longest Univalue Path.md

687. Longest Univalue Path

Question:

Given a binary tree, find the length of the longest path where each node in the path has the same value. This path may or may not pass through the root.

Note: The length of path between two nodes is represented by the number of edges between them.

Example 1:

Input:

              5
             / \
            4   5
           / \   \
          1   1   5

Output:

2

Example 2:

Input:

              1
             / \
            4   5
           / \   \
          4   4   5

Output:

2

Note: The given binary tree has not more than 10000 nodes. The height of the tree is not more than 1000.

Solution:

• Method 1: Divide and conquer(Recursion).

  /**
   * Definition for a binary tree node.
   * public class TreeNode {
   *     int val;
   *     TreeNode left;
   *     TreeNode right;
   *     TreeNode(int x) { val = x; }
   * }
   */
  class Solution {
      private int result;
      public int longestUnivaluePath(TreeNode root) {
          if(root == null) return 0;
          getPath(root);
          return this.result - 1;
      }
      private int getPath(TreeNode node){
          if(node == null) return 0;
          int left = getPath(node.left);
          int right = getPath(node.right);
          int res = 1 + ((node.left == null || node.left.val == node.val) ? left: 0)
              + ((node.right == null || node.right.val == node.val) ? right: 0);
          this.result = Math.max(result, res);
          return Math.max((node.left == null || node.left.val == node.val) ? left: 0, 
                         (node.right == null || node.right.val == node.val) ? right: 0) + 1;
      }
  }