力扣hot100 - 二叉树的直径

题目：思路：经过某一个节点的最大长度为 左子树高度 + 右子树高度。我们从下往上遍历，计算左子树和右子树的高度，在这个过程中不断更新最大长度。

/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode() {} * TreeNode(int val) { this.val = val; } * TreeNode(int val, TreeNode left, TreeNode right) { * this.val = val; * this.left = left; * this.right = right; * } * } */ class Solution { public int maxlength = 0; public int diameterOfBinaryTree(TreeNode root) { dfs(root); return maxlength; } public int dfs(TreeNode root){ if(root == null) return 0; int l = dfs(root.left); int r = dfs(root.right); maxlength = Math.max(l + r , maxlength); return Math.max(l , r) +1; } }