Create next_greater_element_i.cpp

Author: avidLearnerInProgress

next_greater_element_i.cpp

@@ -0,0 +1,49 @@
+class Solution {
+public:
+    vector<int> nextGreaterElement(vector<int>& nums1, vector<int>& nums2) {
+// Forward method
+//         unordered_map<int, int> ump; //to keep track of nge for each element in nums2
+//         //map of current element in stack => its nge
+//         stack<int> st; //to store right part of search
+        
+//         for(auto num : nums2) {
+//             while(!st.empty() and st.top() < num) {
+//                 ump[st.top()] = num;
+//                 st.pop();
+//             } 
+//             st.push(num);
+//         }
+        
+//         vector<int> result;
+//         for(auto num : nums1) {
+//             if(ump.find(num) != ump.end()) result.push_back(ump[num]);
+//             else result.push_back(-1);
+//         }
+        
+//         return result;
+
+        //traverse backward
+        if(nums1.size() == 0) return {};
+        
+        int n = nums2.size();
+        vector<int> result;
+        
+        stack<int> st;
+        unordered_map<int, int> ump;
+        
+        for(int i=n-1; i>=0; i--) {
+            
+            while(!st.empty() and st.top() < nums2[i]) st.pop();
+            if(st.empty()) ump[nums2[i]] = -1;    
+            else ump[nums2[i]] = st.top();
+            
+            st.push(nums2[i]);
+        }
+        
+        int m = nums1.size();
+        for(auto num : nums1)
+            result.push_back(ump[num]);
+        
+        return result;
+    }
+};