leetcode解题之543. Diameter of Binary Tree Java版 (二叉树的最大直径）

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 thelongest 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    

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.

• 二叉树的直径：二叉树中从一个结点到另一个节点最长的路径，叫做二叉树的直径
• 采用分治和递归的思想：根节点为root的二叉树的直径 = Max(左子树直径，右子树直径，左子树的最大深度（不包括根节点）+右子树的最大深度（不包括根节点）+1)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {

	// 全局变量，记录最大直径
	int diameter = 0;

	public int diameterOfBinaryTree(TreeNode root) {
		getDepth(root);
		return diameter;
	}

	// 此函数是返回树的最大深度
	private int getDepth(TreeNode root) {
		if (root == null)
			return 0;
		int l = getDepth(root.left);
		int r = getDepth(root.right);
		diameter = Math.max(diameter, l + r);
		return Math.max(l, r) + 1;
	}
}