104. Maximum Depth of Binary Tree

题目：

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.

Hide Tags

 Tree Depth-first Search

 

链接： http://leetcode.com/problems/maximum-depth-of-binary-tree/

题解：

也是DFS。 假如不使用recursive，可以用level order traversal，使用queue来辅助，每增加一层，depth + 1。 Recursive很简单, 有机会要试试iterative

Time Complexity - O(n)， Space Complexity - O(n)。

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

Update

/**
 * 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) {
        if(root == null)
            return 0;
        return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
    }
}

二刷：

Java:

Recursive:

/**
 * 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) {
        if (root == null) {
            return 0;
        }
        return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
    }
}

层序遍历:

/**
 * 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) {
        if (root == null) {
            return 0;
        }
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        int curLevel = 1;
        int nextLevel = 0;
        int depth = 0;
        while (!q.isEmpty()) {
            TreeNode node = q.poll();
            curLevel--;
            if (node.left != null) {
                q.offer(node.left);
                nextLevel++;
            }
            if (node.right != null) {
                q.offer(node.right);
                nextLevel++;
            }
            if (curLevel == 0) {
                curLevel = nextLevel;
                nextLevel = 0;
                depth++;
            }
        }
        return depth;
    }
}

三刷：

/**
 * 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) {
        if (root == null) {
            return 0;
        }
        return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
    }
}