二叉树的最大深度

  1
 / \ 
2   3
   / \
  4   5

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: An integer.
     */
    public int maxDepth(TreeNode root) {
        if (root == null)
            return 0;
            
        int left = maxDepth(root.left);
        int right = maxDepth(root.right);
        return Math.max(left, right) + 1;
    }
    
    //简化写法的话，其实可以一行代码解决：
    // return root == null ? 0 : Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
}

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: An integer.
     */
    public int maxDepth(TreeNode root) {
        if (root == null)
            return 0;
            
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        int depth = 0;  //表示二叉树的深度
        int count = 0;  //表示已从队列中取出当前层的个数
        int amount = 1; //表示当前节点的兄弟节点个数
        
        while (queue.size() != 0) {
            TreeNode top = queue.poll();
            count++;
            if (top.left != null) {
                queue.add(top.left);
            }
            
            if (top.right != null) {
                queue.add(top.right);
            }    
            
            if (count == amount) {
                amount = queue.size();
                count = 0;
                depth++;
            }
        }
        return depth;
    }
}