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.
Note: A leaf is a node with no children.
Example:
Given binary tree [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its minimum depth = 2.
二叉树的经典问题之最小深度问题就是就最短路径的节点个数，还是用深度优先搜索DFS来完成，万能的递归啊。首先判空，若当前结点不存在，直接返回0。然后看若左子结点不存在，那么对右子结点调用递归函数，并加1返回。反之，若右子结点不存在，那么对左子结点调用递归函数，并加1返回。若左右子结点都存在，则分别对左右子结点调用递归函数，将二者中的较小值加1返回即可，参见代码如下：
解法一：
class Solution {
public:
int minDepth(TreeNode* root) {
if (!root) return ;
if (!root->left) return + minDepth(root->right);
if (!root->right) return + minDepth(root->left);
return + min(minDepth(root->left), minDepth(root->right));
}
};
我们也可以是迭代来做，层序遍历，记录遍历的层数，一旦我们遍历到第一个叶结点，就将当前层数返回，即为二叉树的最小深度，参见代码如下：
解法二：
class Solution {
public:
int minDepth(TreeNode* root) {
if (!root) return ;
int res = ;
queue<TreeNode*> q{{root}};
while (!q.empty()) {
++res;
for (int i = q.size(); i > ; --i) {
auto t = q.front(); q.pop();
if (!t->left && !t->right) return res;
if (t->left) q.push(t->left);
if (t->right) q.push(t->right);
}
}
return -;
}
};
类似题目：
Binary Tree Level Order Traversal
参考资料：
https://leetcode.com/problems/minimum-depth-of-binary-tree/
https://leetcode.com/problems/minimum-depth-of-binary-tree/discuss/36153/My-concise-c%2B%2B-solution