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 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) {        ArrayList
     
       prev = new ArrayList
      
       ();                if(root!=null) prev.add(root);        int depth=0;        while(!prev.isEmpty()){            ArrayList
       
         temp = new ArrayList
        
          ();            for(TreeNode node:prev){                if(node.left!=null) temp.add(node.left);                if(node.right!=null) temp.add(node.right);                if(node.left==null && node.right ==null){                    depth++;                    return depth;                }            }                prev = new ArrayList
         
          (temp);            depth++;        }        return depth;    }}