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.

思路：用BFS

public class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null) return 0;
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        int count = 0;
        while (!queue.isEmpty()){
            int level = queue.size();
            for (int i=0;i<level;i++){
                TreeNode node = queue.remove();
                if (node.left!=null){
                    queue.add(node.left);
                }
                if (node.right!=null){
                    queue.add(node.right);
                }
            }
            count++;
        }
        return count;
    }
}