find the starting and ending position of a given target value

HacktoberFest2021/C++ Programs/findOccur.cpp

@@ -0,0 +1,77 @@
+#include<iostream>
+#include<algorithm>
+
+/*  Given an array of integers nums sorted in ascending order, find the starting and ending position of a given target value.
+
+If target is not found in the array, return [-1, -1].
+
+You must write an algorithm with O(log n) runtime complexity.
+
+
+Example 1:
+Input: nums = [5,7,7,8,8,10], target = 8
+Output: [3,4]
+
+Example 2:
+Input: nums = [5,7,7,8,8,10], target = 6
+Output: [-1,-1]
+
+Example 3:
+Input: nums = [], target = 0
+Output: [-1,-1] 
+*/
+
+//time complexity O(logn)
+using namespace std;
+int firstOccur(int arr[],int n,int target){
+ int lo=0;
+ int hi=n-1;
+ int mid=0;
+ int result=-1;
+ while(lo<=hi){
+     mid=lo+(hi-lo)/n;
+     if(arr[mid]==target){
+        result=mid;
+        hi=mid-1; 
+     }
+     else if(arr[mid]<target)
+       lo=mid+1;
+    else
+    hi=mid-1;
+ }
+ return result;
+}
+int lastOccur(int arr[],int n,int target){
+ int lo=0;
+ int hi=n-1;
+ int mid=0;
+ int result=-1;
+ while(lo<=hi){
+     mid=lo+(hi-lo)/n;
+     if(arr[mid]==target){
+        result=mid;
+        lo=mid+1; 
+     }
+     else if(arr[mid]<target)
+       lo=mid+1;
+    else
+    hi=mid-1;
+ }
+ return result;
+
+}
+int main(){
+    int n;
+    cout<<"Enter size of array"<<endl;
+    cin>>n;
+    int arr[n];
+    cout<<"Enter elements in array"<<endl;
+    for(int i=0;i<n;i++)
+     cin>>arr[i];
+    int target;
+    cout<<"Enter the target value"<<endl;
+    cin>>target;
+    int first =firstOccur(arr,n,target);
+    int last=lastOccur(arr,n,target);
+    cout<<first<<" "<<last<<endl;
+}