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;
}
}