Leetcode 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.

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 解题思路：

方法一： 递归解法

方法二： BFS, 求layer 的数目。

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Java code:

递归解法：

public int maxDepth(TreeNode root) {
        //use DFS recursive
        if(root == null) {
            return 0;
        }
        int leftmax = maxDepth(root.left);
        int rightmax = maxDepth(root.right);
        return Math.max(leftmax, rightmax) + 1;   
    }

BFS, 求layer 的数目

public int maxDepth(TreeNode root) {
        //use BFS
        if(root == null){
            return 0;
        }
        int depth = 0;
        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        int curNum = 1; //num of node left in current level
        int nextNum = 0; // num of nodes in next level
        while(!queue.isEmpty()) {
            TreeNode n = queue.poll(); //Retrieves and removes the head (first element) of this list.
            curNum--;
            if(n.left != null) {
                queue.add (n.left);
                nextNum++;
            }
            if(n.right != null) {
                queue.add(n.right);
                nextNum++;
            }
            if(curNum == 0) {
                curNum = nextNum;
                nextNum = 0;
                depth++;
            }
        }
        return depth;
    }

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Reference:

1. http://www.cnblogs.com/springfor/p/3879619.html