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.

 题目含义：找出二叉树的深度

方法一：递归

    public int maxDepth(TreeNode root) {
        return root == null ? 0 : Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
    }

方法二：BFS

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

方法三：

    public int maxDepth(TreeNode root) {
        if(root == null)  return 0;
        Stack<TreeNode> stack = new Stack<>();
        Stack<Integer> value = new Stack<>();
        stack.push(root);
        value.push(1);
        int max = 0;
        while(!stack.isEmpty()) {
            TreeNode node = stack.pop();
            int temp = value.pop();
            max = Math.max(temp, max);
            if(node.left != null) {
                stack.push(node.left);
                value.push(temp+1);
            }
            if(node.right != null) {
                stack.push(node.right);
                value.push(temp+1);
            }
        }
        return max;
    }