687. Longest Univalue Path 最长的唯一路径

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.

给定二叉树，找到路径中每个节点具有相同值的最长路径的长度。此路径可能通过或不能通过根。

let max = 0;
var longestUnivaluePath = function(root) { 
  max = 0;
  helper(root, null);
  return max;
};
var helper = (node,parentVal) => { 
  if (!node) return 0;
  let left = helper(node.left, node.val);
  let right = helper(node.right, node.val);
  max = Math.max(left + right, max);
  if (node.val == parentVal) {
    return Math.max(left, right) + 1;
  } else { 
    return 0;
  }
}

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