111 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：

input：[3,9,20,null,null,15,7]
output：2

Note：

A leaf is a node with no children.

解释下题目：

返回一棵二叉树到子树最小的距离

1. DFS

实际耗时：0ms

    public int minDepth(TreeNode root) {
        return getMinDepth(root);
    }

    public int getMinDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int left = getMinDepth(root.left);
        int right = getMinDepth(root.right);
        if (left == 0) {
            return right + 1;
        }
        if (right == 0) {
            return left + 1;
        }

        int min = Math.min(left, right);
        return min + 1;
    }

踩过的坑：[1,2] 应该输出2

  思路很简单，就是有一点我第一次做的时候忽略了，就是如果一个节点要都没有左子树和右子树才是到达最下面了，否则还需要走，我就是错在这里

时间复杂度O(n)

空间复杂度O(1)