Merge pull request #717 from Karnika06/Devincept8

Added Next greater element using stack problem

Author: smv1999

Data Structures/Stacks/Next_Greater_Element.cpp

@@ -0,0 +1,97 @@
+/*
+Implementation of Next Greater Element problem using stack
+
+Given an array, print the Next Greater Element (NGE) for every element. 
+The Next greater Element for an element x is the first greater element on 
+the right side of x in array. Elements for which no greater element exist, 
+consider next greater element as -1.
+
+For the input array [4, 5, 2, 25}, the next greater elements for each element are as follows.
+
+Element --> NGE
+
+4 --> 5
+5 --> 25
+2 --> 25
+25 --> -1    */
+
+#include <cmath>
+#include <cstdio>
+#include <vector>
+#include <iostream>
+#include <algorithm>
+#include <stack>
+#include <bits/stdc++.h>
+using namespace std;
+
+vector<int> NGE(vector<int> v)
+{
+    vector<int> nge(v.size());
+    stack<int> stk;
+    for(int i=0;i<v.size(); i++)
+    {
+        while(!stk.empty() && v[i] > v[stk.top()])
+		{
+            nge[stk.top()]=i;
+            stk.pop();
+        }
+        stk.push(i);
+    }
+    while(!stk.empty())
+    {
+        nge[stk.top()]=-1;
+            stk.pop();
+    }
+    return nge;
+}
+
+int main() 
+{
+    int n;
+    cin >> n;
+    vector<int> v(n);
+    
+    for(int i=0;i<n;i++)
+        cin >> v[i];
+        
+    vector<int> nge= NGE(v);
+    
+    for(int i=0; i<n; i++)
+        cout<< v[i] << " " << (nge[i]== -1 ? -1 : v[nge[i]]) << endl;
+        
+    return 0;
+}
+
+/*
+
+Test case 1:
+
+Sample Input
+
+4
+4 5 2 25
+
+Sample Output
+
+4 5
+5 25
+2 25
+25 -1
+
+Test case 2:
+
+Sample Input
+
+4
+13 7 6 12
+
+Sample Output
+
+13 -1
+7 12
+6 12
+12 -1
+
+Time Complexity: O(n)
+Space Complexity: O(n)
+*/