/**
 * 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) {

        if( root == null){
            return 0;
        }

        Queue queue = new LinkedList<TreeNode>();     
        queue.add(root);
        int dep = 1;
        while( !queue.isEmpty() ){
            int size = queue.size();
            for( int i = 0;i<size;i++){
                TreeNode node = (TreeNode) queue.poll();
                if( node.left == null && node.right == null )
                    return dep;
                if( node.left != null)
                    queue.add(node.left);
                if( node.right != null)
                    queue.add(node.right);

            }
            dep++;
        }
        return dep;

    }
}