543. Diameter of Binary Tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
 
 // 找从左到每个node的最长距离 和 从右到每个node的最长距离
public class Solution {
    public int diameterOfBinaryTree(TreeNode root) {
        if(root == null){
            return 0;
        }
        int maxAns = 0;
        // traverse every node
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while (!stack.empty()) {
            TreeNode node = stack.pop();
            int temp = maxDepth(node.left) + maxDepth(node.right);
            maxAns = maxAns > temp? maxAns: temp;
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }
        }
        return maxAns;
    }
    
    public int maxDepth(TreeNode root){
        if(root == null){
            return 0;
        }
        return Math.max(maxDepth(root.left), maxDepth(root.right) ) + 1;
    }
}