/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int maxDepth(TreeNode root) {
        // 递归方法
        // return root == null? 0:Math.max(maxDepth(root.left),maxDepth(root.right))+1;
        
        // 非递归方法
        if(root == null) {
            return 0;
        }
        Queue<TreeNode> q = new LinkedList();
        q.offer(root);
        int precount = 1;
        int pcount = 0;
        int level = 0;
        
        while(!q.isEmpty()) {
            TreeNode t = q.poll();
            precount --;
            if(t.left != null) {
                q.offer(t.left);
                pcount++;
            }
            if(t.right != null) {
                q.offer(t.right);
                pcount++;
            }
            if(precount == 0) {
                precount = pcount;
                pcount = 0;
                level++;
            }
        }
        return level;
    }
}