给定二叉树，找到它的最小深度。

给定二叉树，找到它的最小深度。最小深度是沿从根节点到最近的叶节点的最短路径上的节点数。
注意：叶子是没有子节点的节点。

package com.example.tree;

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class SolutionMinDepth {

    public static int minDepth(TreeNode root) {
        if(root == null){
            return 0;
        }

        int leftMin = minDepth(root.left);
        int rightMin = minDepth(root.right);

        return (leftMin == 0 || rightMin == 0) ? leftMin + rightMin + 1: Math.min(leftMin,rightMin) + 1;
    }

    public static void main(String[] args) {
        TreeNode root = new TreeNode(3);
        TreeNode t1 = new TreeNode(9);
        TreeNode t2 = new TreeNode(20);
        TreeNode t5 = new TreeNode(15);
        TreeNode t6 = new TreeNode(7);

        root.left = t1;
        root.right = t2;
        t2.left = t5;
        t2.right = t6;

        System.out.println(minDepth(root));
    }

}