求二叉树的最小深度

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.

思路：利用递归

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     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)
            return run(root.right)+1;
        if(root.right==null)
            return run(root.left)+1;
        return Math.min(run(root.left),run(root.right))+1;
    }
}