Merge pull request #644 from neha030/dev15

Previous Greater Element added

Author: Saviour1001

Data Structures/Stacks/Previous_Greater_Element.cpp

@@ -0,0 +1,69 @@
+/*
+PREVIOUS GREATER ELEMENT USING STACK
+
+An Efficient Approach to find Previous Greater Element for every 
+element in an array. Given an array of Distinct Integers, find closest 
+(Positive-Wise) greater on left of every element. If there is no 
+greater element on left then print -1.
+*/
+
+#include <bits/stdc++.h>
+using namespace std;
+
+/* This function is used to check if top of the 
+stack is smaller we remove it or if top is 
+greater, then this element is previous greater
+element and print it.
+*/
+
+void printPrevGreater(int arr[],int n){
+    //Stack to store Previous Greater on top of stack.
+    stack<int>s;
+
+    /*Store first element of array to 
+    check if top item is smaller then
+    current item.*/
+    s.push(arr[0]);
+
+    /* This loop will help to find the 
+       previous greater element from 
+       the current value and simply print
+       it and if not found print -1.
+       NOTE : After every iteration whaterever the remaining element is 
+       present at the top is Previous Greater and if stack is empty
+       then previous Greater is -1. 
+    */
+    for(int i=0;i<n;i++){
+        while(s.empty()==false && s.top()<=arr[i])
+            s.pop();
+
+        //pg : Previous Greater
+        int pg=s.empty()?-1:s.top();
+        cout<<pg<<" ";
+        s.push(arr[i]);
+    }
+}
+// Driver's Code
+int main() 
+{ 
+    int n;
+    cout<<"Accept the value : ";
+    cin>>n;
+    int arr[n];
+    cout<<"Accept the array elements : ";
+    for(int i=0;i<n;i++){
+        cin>>arr[i];
+    }
+    cout<<"Next Greater Elements : ";
+    printPrevGreater(arr,n);
+    return 0; 
+}
+/*
+INPUT AND OUTPUT
+Accept the value : 5
+Accept the array elements : 20 30 10 5 15
+
+Next Greater Elements : -1  -1  30  10 30
+
+Time Complexity: O(n)
+*/