題目：

給定二叉樹，找到它的最小深度。

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

最小深度是沿從根節點到最近的葉節點的最短路徑上的節點數。

Note: A leaf is a node with no children.

注意：葉子是沒有子節點的節點。

Example:

二叉樹： [3,9,20,null,null,15,7],

3

/ \

9 20

/ \

15 7

return its minimum depth = 2.

public class Minimumdepth {
    public class TreeNode{
        TreeNode left;
        TreeNode right;
        int val;
        public void TreeNode(int val){
            this.val = val;
        }
    }
    public class Solution {
        public int run(TreeNode root) {
            if(root == null){
                return 0;
            }
            int left = run(root.left);
            int right = run(root.right);
            return (left == 0 || right == 0) ? left + right + 1 : Math.min(left, right) + 1;
        }
    }
    public static void main(String[] args) {

    }
}