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.

Solution1: recursion
Run time complexity is O(n), stack space.

/**
 * Definition for binary tree
 * 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;
        else
            return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
    }
}

Solution2: iterative
BFS。一直走到最后再返回深度。

Run time complexity is O(n), space complexity is O(n).

/**
 * Definition for binary tree
 * 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;
        
        LinkedList queue = new LinkedList();
        int lastNum = 1;
        int curNum = 0;
        int level = 0;
        queue.offer(root);
        while(!queue.isEmpty()) {
            TreeNode cur = queue.poll();
            lastNum--;
            if(cur.left != null) {
                queue.offer(cur.left);
                curNum++;
            }
            if(cur.right != null) {
                queue.offer(cur.right);
                curNum++;
            }
            if(lastNum == 0) {
                lastNum = curNum;
                curNum = 0;
                level++;
            }
        }
        return level;
    }
}