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.
思路：
递归，注意是到leaf node，所以有一个孩子为空的话，则取非空的那一孩子
package tree;
public class MinimumDepthOfBinaryTree {
public int minDepth(TreeNode root) {
if (root == null) return 0;
if (root.left == null || root.right == null) return root.left == null ? (1 + minDepth(root.right)) : (1 + minDepth(root.left));
return Math.min(1 + minDepth(root.left), 1 + minDepth(root.right));
}
}