[LeetCode]111. Minimum Depth of Binary Tree

题目描述：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.

分析：查找一棵二叉树从根节点到叶子节点的最短距离

解题思路:类似于求二叉树的深度，但是因为是求二叉树的最小深度，所以返回的是左子树和右子树中最小的深度。

public int minDepth(TreeNode root) {
if(root==null)return 0;
if(root.left==null)return minDepth(root.right)+1;
if(root.right==null)return minDepth(root.left)+1;
return Math.min(minDepth(root.left), minDepth(root.right))+1;
}