543. Diameter of Binary Tree

对于任意节点node，可以计算其左子树的深度left和右子树的深度right，left+right+1即为过node的最长距离。遍历树的节点，取最大值即可：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int ans=1;
    public int diameterOfBinaryTree(TreeNode root) {
        // if(root==null){
        //     return 0;
        // }
        helper(root);
        return ans-1;
    }
    public int helper(TreeNode node){  //获取树的深度
        if(node==null){
            return 0;
        }
        int left = helper(node.left);
        int right = helper(node.right);
        ans = Math.max(ans,left+right+1);
        return Math.max(left,right)+1;
    }
}