diff --git a/solutions/398. Random Pick Index.md b/solutions/398. Random Pick Index.md
new file mode 100644
index 0000000..a9ca97f
--- /dev/null
+++ b/solutions/398. Random Pick Index.md	
@@ -0,0 +1,33 @@
+# [398. Random Pick Index](https://leetcode.com/problems/random-pick-index/)
+
+# 思路
+给定一个包含重复数字的数组和一个target，要求随机返回一个target在数组中的下标。
+题目指明我们不能用太多的空间，那我们可以考虑蓄水池采样（Reservoir Sampling）算法，这个算法我已经在[382题解](https://github.com/ShusenTang/LeetCode/blob/master/solutions/382.%20Linked%20List%20Random%20Node.md)中详细说过了，
+这里不再赘述，直接给出代码。
+
+pick时间复杂度O(n)，空间复杂度O(1)
+
+# C++
+```C++
+class Solution {
+private:
+    vector<int>::iterator begin;
+    int size;
+public:
+    Solution(vector<int>& nums) {
+        begin = nums.begin();
+        size = nums.size();
+    }
+    
+    int pick(int target) {
+        int res = -1, count = 1;
+        
+        for(int i = 0; i < size; i++){
+            if(*(begin+i) != target) continue;
+            if(rand() % count == 0) res = i;
+            count++;
+        }
+        return res;
+    }
+};
+```