题目描述：
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.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;        //数据值
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int run(TreeNode root) {
        if(root==null){
            return 0;
        }
        if(root.left==null&&root.right==null){
            return 1;
        }
        if(root.left!=null&&root.right!=null){
            int l=run(root.left);
            int r=run(root.right);
            return (l<r?l:r)+1;
        }
        if(root.left!=null){
            return run(root.left)+1;
        }else if(root.right!=null){
            return run(root.right)+1;
        }
        return 0;
    }
}