diff --git a/OJ notes/pages/Leetcode Next-Greater-Element-I.md b/OJ notes/pages/Leetcode Next-Greater-Element-I.md
index 1c62aca..7644ee7 100644
--- a/OJ notes/pages/Leetcode Next-Greater-Element-I.md	
+++ b/OJ notes/pages/Leetcode Next-Greater-Element-I.md	
@@ -68,4 +68,36 @@ Stack is FILO, which means, when iterating from the last element, if you push a
 
 ### Solution
 
-```cpp
\ No newline at end of file
+```cpp
+class Solution {
+public:
+  vector<int> nextGreaterElement(vector<int> &nums1, vector<int> &nums2) {
+    // Hash table to link the subsets.
+    unordered_map<int, int> greater;
+    stack<int> st;
+
+    for (int i = nums2.size() - 1; i >= 0; i--) {
+      while (!st.empty() && st.top() < nums2[i]) {
+        st.pop();
+      }
+      if (!st.empty()) {
+        greater[nums2[i]] = st.top();
+      }
+
+      st.push(nums2[i]);
+    }
+
+    // double pointers can't be used, because nums doesn't have a order.
+    vector<int> ans(nums1.size());
+    for (int i = 0, size1 = nums1.size(); i < size1; i++) {
+      if (greater.find(nums1[i]) != greater.end()) {
+        ans[i] = greater[nums1[i]];
+      } else {
+        ans[i] = -1;
+      }
+    }
+
+    return ans;
+  }
+};
+```
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