剑指 Offer 55

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
//方法一、递归法，后序遍历

class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null) return 0;

        int nleft = maxDepth(root.left);
        int nright = maxDepth(root.right);
        return (nleft > nright) ? nleft+1 : nright+1;
    }
}

 方法二、非递归，（层次遍历）

public int maxDepth(TreeNode root) {
        if (root == null) return 0;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int res = 0;
        while (!queue.isEmpty()) {
            res++;
            int n = queue.size();
            for (int i = 0; i < n; i++) {
                TreeNode node = queue.poll();
                if (node.left != null) queue.add(node.left);
                if (node.right != null) queue.add(node.right);
            }
        }
        return res;
    }