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.

Example

Given a binary tree as follow:

  1
 / \ 
2   3
   / \
  4   5

The minimum depth is 2.

分析：

这题看似简单，但是你写完程序run了以后发现通不过，再读一遍问题发现理解有错。如果上面的树没有2，可能很多人算出来的结果是1，因为左子树为空。但是实际上minimum depth 是 3. 意思就是说，只有一个node的左右节点都为空，我们才能认为这是结束点。

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: An integer.
     */
    public int minDepth(TreeNode root) {
        if (root == null) return 0;
        
        if (root.left == null && root.right == null) return 1;
        
        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;
    }
}