二叉树的深度

解法一:递归

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

    }
}

解法二：层序遍历的思想

/**
 * 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) {
        if(root == null) return 0;

        List<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int res = 0;
        while(!queue.isEmpty()) {
            List<TreeNode> tmp = new LinkedList<>();
            for(TreeNode node : queue) {
                if(node.left != null) tmp.add(node.left);
                if(node.right != null) tmp.add(node.right);
            }
            queue = tmp;
            res++;
        }
        return res;
    }
}

层序遍历思路总结：