Minimum Depth of Binary Tree

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.

Solution:
Basic tree traverse question.

/**
 * 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;
        return helper(root, 0);
    }
    public int helper(TreeNode root, int depth){
        if(root.left == null  && root.right == null){
            return depth + 1;  
        } 
        int min = Integer.MAX_VALUE;
        if(root.left != null) min = Math.min(min, helper(root.left, depth + 1));
        if(root.right != null) min = Math.min(min, helper(root.right, depth + 1));
        return min;
    }
}