104. Maximum Depth of Binary Tree

这道题和leetcode第102题没啥区别，通过对树进行遍历即可。设置一个list集合，每向下一层，就给该层编一个号，将编号放入集合。当层数大于等于list元素个数（总的层数）时，需要新加一个元素到list中，如果不是，继续遍历即可。程序如下：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
   public int maxDepth(TreeNode root) {
		  List<Integer> re=new ArrayList<>();
		  if(root==null)
			  return 0;
		  helper(re,root,0);
		  return re.size();
	        
	    }
	  public void helper(List<Integer> list,TreeNode root,int height) {
		  if(root==null)
			  return;
		  if(height>=list.size())
			  list.add(height);
		  helper(list,root.left,height+1);
		  helper(list,root.right,height+1);
		  
	  }
}

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int maxDepth(TreeNode node) {
        if(node == null)
            return 0;
        if(node.left == null && node.right == null) {
            return 1;
        }
        int leftDepth = maxDepth(node.left);
        int rightDepth = maxDepth(node.right);
        return Math.max(leftDepth, rightDepth)+1;
    }
}

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null) return 0;
        // return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        q.offer(root);
        int depth = 0;
        while(q.peek() != null){
            depth++;
            int len = q.size();
            for(int i = 0; i < len; i++){
                TreeNode node = q.poll();
                if(node.left != null) q.offer(node.left);
                if(node.right != null) q.offer(node.right);
            }
        }
        
        return depth;
        
        
        
    }
}