/**
 * Definition for binary tree
 * 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;
        
        if(root.left == null && root.right == null) 
            return 1;
        
        int leftDepth = minDepth(root.left);
        int rightDepth = minDepth(root.right);
        
        if(leftDepth == 0){
            return rightDepth + 1;
        }else if(rightDepth == 0){
            return leftDepth + 1;
        }else{
            return Math.min(leftDepth, rightDepth)+1;
        }
    }
}