Create f21

f21

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+// A Stack based C++ program to find next 
+// greater element for all array elements 
+// in same order as input. 
+#include <bits/stdc++.h> 
+
+using namespace std; 
+
+/* prints element and NGE pair for all 
+elements of arr[] of size n */
+void printNGE(int arr[], int n) 
+{ 
+	stack<int> s; 
+	unordered_map<int, int> mp; 
+
+	/* push the first element to stack */
+	s.push(arr[0]); 
+	
+
+	// iterate for rest of the elements 
+	for (int i = 1; i < n; i++) { 
+
+		if (s.empty()) { 
+			s.push(arr[i]); 
+			continue; 
+		} 
+
+		/* if stack is not empty, then 
+	pop an element from stack. 
+	If the popped element is smaller 
+	than next, then 
+	a) print the pair 
+	b) keep popping while elements are 
+	smaller and stack is not empty */
+		while (s.empty() == false && s.top() < arr[i]) { 
+			mp[s.top()] = arr[i]; 
+			s.pop(); 
+		} 
+
+		/* push next to stack so that we can find 
+	next smaller for it */
+		s.push(arr[i]); 
+	} 
+
+	/* After iterating over the loop, the remaining 
+elements in stack do not have the next smaller 
+element, so print -1 for them */
+	while (s.empty() == false) { 
+		mp[s.top()] = -1; 
+		s.pop(); 
+	} 
+
+	for (int i=0; i<n; i++) 
+	cout << arr[i] << " ---> " << mp[arr[i]] << endl; 
+} 
+
+/* Driver program to test above functions */
+int main() 
+{ 
+	int arr[] = { 11, 13, 21, 3 }; 
+	int n = sizeof(arr) / sizeof(arr[0]); 
+	printNGE(arr, n); 
+	return 0; 
+}