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+# [111. Minimum Depth of Binary Tree](https://leetcode.com/problems/minimum-depth-of-binary-tree/description/)
+# 思路
+求二叉树的最小深度。依次分为以下情况:
+* 若为空树，则返回0；
+* 否则，若左右子树都为空，返回1；
+* 否则，若左子树为空，那么最小深度就为右子树的最小深度加1；若右子树为空，那么最小深度就为左子树的最小深度加1；
+* 否则(即左右子树都不空)，那么最小深度就是min(1+左子树最小深度, 1+右子树最小深度)；
+
+# C++
+``` C++
+class Solution {
+public:
+    int minDepth(TreeNode* root) {
+        if(root == NULL) return 0;
+        else if(root -> left == NULL && root -> right == NULL) return 1;
+        else if(root -> left == NULL) return 1 + minDepth(root -> right);
+        else if(root -> right == NULL) return 1 + minDepth(root -> left);
+        return 1 + min(minDepth(root -> left), minDepth(root -> right));
+    }
+};
+```