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.

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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));
}
}