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

1     public int maxDepth(TreeNode root) {
2         return root == null ? 0 : Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
3     }

方法二：BFS

 1     public int maxDepth(TreeNode root) {
 2         if(root == null) return 0;       
 3         Queue<TreeNode> queue = new LinkedList<>();
 4          queue.offer(root);
 5          int count = 0;
 6          while(!queue.isEmpty()) {
 7              int size = queue.size();
 8              for (int i=size;i>0;i--)
 9              {
10                  TreeNode node = queue.poll();
11                  if(node.left != null)  queue.offer(node.left);
12                  if(node.right != null) queue.offer(node.right);
13              }
14              count++;
15          }
16          return count;
17     }

方法三：

 1     public int maxDepth(TreeNode root) {
 2         if(root == null)  return 0;
 3         Stack<TreeNode> stack = new Stack<>();
 4         Stack<Integer> value = new Stack<>();
 5         stack.push(root);
 6         value.push(1);
 7         int max = 0;
 8         while(!stack.isEmpty()) {
 9             TreeNode node = stack.pop();
10             int temp = value.pop();
11             max = Math.max(temp, max);
12             if(node.left != null) {
13                 stack.push(node.left);
14                 value.push(temp+1);
15             }
16             if(node.right != null) {
17                 stack.push(node.right);
18                 value.push(temp+1);
19             }
20         }
21         return max;
22     }