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.
1 public int maxDepth(TreeNode root) {
2 if(root==null)
3 return 0;
4 int leftmax = maxDepth(root.left);
5 int rightmax = maxDepth(root.right);
6 return Math.max(leftmax, rightmax)+1;
7 }
非递归解法，参考codeganker（http://codeganker.blogspot.com/2014/02/maximum-depth-of-binary-tree-leetcode.html）的：
1 public int maxDepth(TreeNode root) {
2 if(root == null)
3 return 0;
4
5 int depth = 0;
6 LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
7 queue.add(root);
8 int curNum = 1; //num of nodes left in current level
9 int nextNum = 0; //num of nodes in next level
10 while(!queue.isEmpty()){
11 TreeNode n = queue.poll();
12 curNum--;
13 if(n.left!=null){
14 queue.add(n.left);
15 nextNum++;
16 }
17 if(n.right!=null){
18 queue.add(n.right);
19 nextNum++;
20 }
21 if(curNum == 0){
22 curNum = nextNum;
23 nextNum = 0;
24 depth++;
25 }
26 }
27 return depth;
28 }
