Minimum Depth of Binary Tree

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.

求一棵树的最短路径，同样用递归来解决。注意的是如果遇到一个节点只有一个孩子，这时我们要继续往下搜，要不然不符合题目的规定，同样的还有处理最大路径的问题，都是采用递归解决。代码如下：
最短路径：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int minDepth(TreeNode root) {
        if(root == null) return 0;
        return minDepth(root, 1);
    }
    public int minDepth(TreeNode root, int depth) {
        if(root == null) return 0;
        if(root.left != null && root.right != null)
            return Math.min(minDepth(root.left, depth + 1), minDepth(root.right, depth + 1));
        if(root.left != null && root.right == null)
            return minDepth(root.left, depth + 1);
        if(root.left == null && root.right != null)
            return minDepth(root.right, depth + 1);
        return depth;
    }
}

最长路径：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null) return 0;
        return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
    }
}