1  /**
 2      * Given a binary tree, find its minimum depth.
 3      * the minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.
 4      * @param root
 5      * @return
 6      */
 7     public int minDepth(TreeNode root){
 8         //the boundary condition
 9         if(root == null)
10             return 0;
11         return minDepthRec(root);
12     }
13     private int minDepthRec(TreeNode root){
14         if(root == null)
15             return 0;
16         if(root.left == null && root.right == null)
17             return 1;
18         //the condition judged here is very important here
19         if(root.left == null)
20             return minDepthRec(root.right)+1;
21         if(root.right == null)
22             return minDepthRec(root.left)+1;
23         return Math.min(minDepthRec(root.left), minDepthRec(root.right))+1;
24     }

 1 /**
 2      * Given a binary tree, find its maximum depth.
 3      * The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.
 4      * @param root
 5      * @return
 6      */
 7     public int maxDepth(TreeNode root){
 8         if(root == null)
 9             return 0;
10         if(root.left == null && root.right == null)
11             return 1;
12         return Math.max(maxDepth(root.left), maxDepth(root.right))+1;
13     }