二叉树的深度

1、递归后序遍历

class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null) return 0;
        return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
    }
}

2、层次遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    Queue<TreeNode> q = new LinkedList<>();
    //层次遍历实现求解
    public int maxDepth(TreeNode root) {
     if(root == null) return 0;
     q.add(root);
     int depth = 0;   
     while(!q.isEmpty()){
         depth++;
         int size = q.size();
         for(int i=0;i<size;i++){
             TreeNode t = q.poll();
             if(t.left != null){
                 q.add(t.left);
             }
             if(t.right != null){
                 q.add(t.right);
             }
         }
     }
     return depth;

    }
}