543 Diameter of Binary Tree

Given a binary tree, you need to compute the length of the diameter of the tree. The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.

Example: Given a binary tree

          1
         / \
        2   3
       / \     
      4   5

Return3, which is the length of the path [4,2,1,3] or [5,2,1,3].

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

这题很简单，就从底向上传现在的高度，然后每次更新一下总的path长度。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    int max = 0;
    public int diameterOfBinaryTree(TreeNode root) {
        if (root == null) {
            return 0;
        }

        helper(root);
        return max;
    }

    private int helper(TreeNode root) {
        if (root == null) {
            return 0;
        }

        int left = helper(root.left);
        int right = helper(root.right);

        int cur = Math.max(left, right) + 1;
        max = Math.max(left + right, max);
        return cur;
    }
}