104. Maximum Depth of Binary Tree

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

这道题用递归比较好做，代码如下：

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public int maxDepth(TreeNode root) {        return root == null? 0: Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;    }}

另外这道题也可以用BFS和DFS来做，代码如下：

DFS

public int maxDepth(TreeNode root) {    if(root == null) {        return 0;    }        Stack<TreeNode> stack = new Stack<>();    Stack<Integer> value = new Stack<>();    stack.push(root);    value.push(1);    int max = 0;    while(!stack.isEmpty()) {        TreeNode node = stack.pop();        int temp = value.pop();        max = Math.max(temp, max);        if(node.left != null) {            stack.push(node.left);            value.push(temp+1);        }        if(node.right != null) {            stack.push(node.right);            value.push(temp+1);        }    }    return max;}// 7ms

BFS

public int maxDepth(TreeNode root) {    if(root == null) {        return 0;    }    Queue<TreeNode> queue = new LinkedList<>();    queue.offer(root);    int count = 0;    while(!queue.isEmpty()) {        int size = queue.size();        while(size-- > 0) {            TreeNode node = queue.poll();            if(node.left != null) {                queue.offer(node.left);            }            if(node.right != null) {                queue.offer(node.right);            }        }        count++;    }    return count;}// 3ms