543. Diameter of Binary Tree

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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

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2
3
4
5
    1
   / \
  2   3
 / \     
4   5

Return 3, 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.

解法1：

用一个function计算每一个node的max path， 然后对每一个node更新最大的diameter，每一个node的diameter是left + right。
C++

1

Java

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    int diameter = 0;
    public int diameterOfBinaryTree(TreeNode root) {
        int temp = helper(root);
        return diameter;
    }
    private int helper(TreeNode root) {
        // Use this function to calculate the longest path rooted at "root"
        if (root == null) {
            return 0;
        }
        if (root.left == null && root.right == null) {
            return 1;
        }
        int left = helper(root.left);
        int right = helper(root.right);
        // Update the longest path
        diameter = Math.max(diameter, left + right);
        return Math.max(left, right) + 1;   // Update the longest path rooted at root
    }
}