Created next_greater.cpp

Stack/next_greater.cpp

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+// A Stack based C++ program to find next
+// greater element for all array elements.
+#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;
+
+	/* 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])
+		{
+			cout << s.top()
+				<< " --> " << arr[i] << endl;
+			s.pop();
+		}
+
+		/* push next to stack so that we can find
+		next greater for it */
+		s.push(arr[i]);
+	}
+
+	/* After iterating over the loop, the remaining
+	elements in stack do not have the next greater
+	element, so print -1 for them */
+	while (s.empty() == false) {
+		cout << s.top() << " --> " << -1 << endl;
+		s.pop();
+	}
+}
+
+/* Driver code */
+int main()
+{
+	int arr[] = { 11, 13, 21, 3 };
+	int n = sizeof(arr) / sizeof(arr[0]);
+	printNGE(arr, n);
+	return 0;
+}