Minimum Depth of Binary Tree

题目：

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

解答：

public calss Solution {

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

        if(root.left == null) {
            return minDepth(root.right);
        }

        if(root.right == null) {
            return minDepth(root.left);
        }

        return Math.min(minDepth(root.left), minDepth(root.right)) + 1;
    }
}