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; } * } */class Solution {    public int minDepth(TreeNode root) {        return null == root ? 0 : (null != root.left && null != root.right) ? Math.min(minDepth(root.left), minDepth(root.right)) + 1 : Math.max(minDepth(root.left), minDepth(root.right)) + 1;    }}