111. Minimum Depth of Binary Tree

Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

代码如下：

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 public class Solution {

     public int minDepth(TreeNode root) {

         Stack<Integer> stack=new Stack<>();
         int depth=0;
        if(root==null)
        return 0;

        if(root.left==null&&root.right==null)
        {return 1;}

        if(root.left!=null)
        {
            depth=1+minDepth(root.left);
             if(stack.empty()||stack.peek()>depth)
            stack.push(depth);
        }

         if(root.right!=null)
        {
            depth=1+minDepth(root.right);
             if(stack.empty()||stack.peek()>depth)
             stack.push(depth);
        }

        return stack.peek();
     }
 }