From 46ed21da2f8420118de6c670194d3ece5827d87e Mon Sep 17 00:00:00 2001
From: Premal Samale <premalsamale@Premals-MacBook-Pro.local>
Date: Mon, 5 Jan 2026 20:42:35 +0530
Subject: [PATCH] Precourse2 completed

---
 Exercise_1.java | 24 +++++++++++++++++++-
 Exercise_2.java | 37 +++++++++++++++++++++++++++---
 Exercise_3.java | 32 ++++++++++++++++++++++++++
 Exercise_4.java | 50 ++++++++++++++++++++++++++++++++++++++++-
 Exercise_5.java | 60 ++++++++++++++++++++++++++++++++++++++++++++++++-
 5 files changed, 197 insertions(+), 6 deletions(-)

diff --git a/Exercise_1.java b/Exercise_1.java
index c3ff1141..d69efcfd 100644
--- a/Exercise_1.java
+++ b/Exercise_1.java
@@ -1,8 +1,30 @@
+// Time Complexity : o(log n)
+// Space Complexity :o(1)
+// Did this code successfully run on Leetcode : N/A it is not a leetcode problem
+// Any problem you faced while coding this :no
+
+
+// Your code here along with comments explaining your approach
+//1. consider left pointer at 0th index and right pointer at last index and middle of both.
+//2.  iterate until left and right not crossing 
+//3. check if target is matching with mid element, if yes return mid index.
+//4. Else if target is greater than mid element then check the target on right side of of mid, else go towards left side.
+
 class BinarySearch { 
     // Returns index of x if it is present in arr[l.. r], else return -1 
     int binarySearch(int arr[], int l, int r, int x) 
     { 
-        //Write your code here
+        while(l<=r){
+            int mid = l+(r-l)/2;
+            if(arr[mid]== x){
+                return mid;
+            }else if(arr[mid]<x){
+                l = mid+1;
+            }else{
+                r = mid-1;
+            }
+        }
+        return -1;
     } 
   
     // Driver method to test above 
diff --git a/Exercise_2.java b/Exercise_2.java
index d0b5fa5f..2304e53c 100644
--- a/Exercise_2.java
+++ b/Exercise_2.java
@@ -1,3 +1,14 @@
+// Time Complexity :o(nlogn)
+// Space Complexity : o(n) recursive stack space, n is no of elements in array.
+// Did this code successfully run on Leetcode : no
+// Any problem you faced while coding this :n/a
+
+
+// Your code here along with comments explaining your approach
+// step 1 decide pivot as last element in array 
+// and partition the element. 
+// recursively call sort on left side of pivot and on right side of pivot.Elements less than pivot are towards left and greater than pivot toward right
+
 class QuickSort 
 { 
     /* This function takes last element as pivot, 
@@ -8,20 +19,40 @@ class QuickSort
        of pivot */
     void swap(int arr[],int i,int j){
         //Your code here   
+        int temp = arr[i];
+        arr[i] = arr[j];
+        arr[j] = temp;
+
     }
     
     int partition(int arr[], int low, int high) 
-    { 
-   	//Write code here for Partition and Swap 
+    {
+        int pivot = arr[high];
+        int i = low-1;
+        for(int j= low;j<=high-1;j++){
+            if(arr[j]<=pivot){
+                i++;
+                swap(arr, i, j);
+            }
+        }
+        swap(arr,i+1,high);
+        return i+1; 	
     } 
+
     /* The main function that implements QuickSort() 
       arr[] --> Array to be sorted, 
       low  --> Starting index, 
       high  --> Ending index */
     void sort(int arr[], int low, int high) 
     {  
-            // Recursively sort elements before 
+        // Recursively sort elements before 
             // partition and after partition 
+            if(low<high){
+                int pivot = partition(arr, low, high);
+                sort(arr, low, pivot-1);
+                sort(arr, pivot+1, high);
+
+            }
     } 
   
     /* A utility function to print array of size n */
diff --git a/Exercise_3.java b/Exercise_3.java
index 1f9b752a..0d9f0732 100644
--- a/Exercise_3.java
+++ b/Exercise_3.java
@@ -1,3 +1,13 @@
+// Time Complexity :o(n)
+// Space Complexity :o(1)
+// Did this code successfully run on Leetcode :n/a
+// Any problem you faced while coding this :n.a
+
+
+// Your code here along with comments explaining your approach
+//Iterate over linklist and find length of linkedList
+//iterate till half of linkedlist and inittialize temp node to head and move temparary node foward.
+//once temparary node reached at half of linkedlist, return temparary node
 class LinkedList 
 { 
     Node head; // head of linked list 
@@ -13,6 +23,16 @@ class Node
             next = null; 
         } 
     } 
+
+    public int getLen(){
+        int length =0;
+        Node temp = head;
+        while(temp!=null){
+            length++;
+            temp = temp.next;
+        }
+        return length;
+    }
   
     /* Function to print middle of linked list */
    //Complete this function
@@ -20,6 +40,18 @@ void printMiddle()
     { 
         //Write your code here
 	//Implement using Fast and slow pointers
+        if(head!=null){
+            int length = getLen();
+            Node temp = head;
+            int middleLength = length/2;
+            while(middleLength!=0){
+                temp = temp.next;
+                middleLength--;
+            }
+            System.out.println("The middle element is [" + temp.data + "]");
+            System.out.println("\n\n");
+        }
+
     } 
   
     public void push(int new_data) 
diff --git a/Exercise_4.java b/Exercise_4.java
index 81afd3c2..15dc287d 100644
--- a/Exercise_4.java
+++ b/Exercise_4.java
@@ -1,11 +1,53 @@
+// Time Complexity : O(nlogn)
+// Space Complexity :O(n)
+// Did this code successfully run on Leetcode : N/A
+// Any problem you faced while coding this :N/A
+
+// Your code here along with comments explaining your approach
+//Divide the array into halves, sort each half recursively, then merge them.
 class MergeSort 
 { 
     // Merges two subarrays of arr[]. 
     // First subarray is arr[l..m] 
     // Second subarray is arr[m+1..r] 
-    void merge(int arr[], int l, int m, int r) 
+    void merge(int arr[], int left, int mid, int right) 
     {  
        //Your code here  
+       int n1 = mid-left+1;
+       int n2 = right - mid;
+       int[] L = new int[n1];
+       int[] R = new int[n2];
+       for(int i =0; i<n1;i++){
+            L[i] = arr[left+i];
+       }
+       for(int j =0; j<n2;j++){
+            R[j] = arr[mid+1+j];
+       }
+       int i = 0; 
+       int j = 0;
+       int k = left;
+       while(i<n1 && j<n2){
+            if(L[i]<=R[j]){
+                arr[k] = L[i];
+                i++;
+            }else{
+                arr[k] = R[j];
+                j++;
+            }
+            k++;
+       }
+       while(i<n1){
+            arr[k] = L[i];
+            i++;
+            k++;
+       }
+       while(j<n2)
+        {
+            arr[k] = R[j];
+            j++;
+            k++;
+        }
+
     } 
   
     // Main function that sorts arr[l..r] using 
@@ -14,6 +56,12 @@ void sort(int arr[], int l, int r)
     { 
 	//Write your code here
         //Call mergeSort from here 
+        if(l<r){
+            int mid = l+(r-l)/2;
+            sort(arr, l,mid);
+            sort(arr, mid+1, r);
+            merge(arr,l, mid, r);
+        }
     } 
   
     /* A utility function to print array of size n */
diff --git a/Exercise_5.java b/Exercise_5.java
index 30e82675..af75f06f 100644
--- a/Exercise_5.java
+++ b/Exercise_5.java
@@ -1,3 +1,21 @@
+import java.util.Stack;
+import java.util.Arrays;
+// Time Complexity : o(logn)
+// Space Complexity :o(logn)
+// Did this code successfully run on Leetcode :no
+// Any problem you faced while coding this : no
+
+
+// Your code here along with comments explaining your approach
+// //Create an empty stack and push first and last element.
+
+// interate over stack and while iterating perform below action:
+// 1. Find pivot
+// 2. push left subarray from first element till pivot
+// 3. push right subarray from next element of pivot till last element.
+
+
+
 class IterativeQuickSort { 
     void swap(int arr[], int i, int j) 
     { 
@@ -7,7 +25,23 @@ void swap(int arr[], int i, int j)
     /* This function is same in both iterative and 
        recursive*/
     int partition(int arr[], int l, int h) 
-    { 
+    {
+        int pivot = arr[h];
+        int i = l-1;
+        for(int j = l; j<h;j++){
+
+            if(arr[j]<=pivot){
+                i++;
+                int temp = arr[i];
+                arr[i] = arr[j];
+                arr[j] = temp;
+            }
+        }
+
+        int temp = arr[i+1];
+        arr[i+1] = arr[h];
+        arr[h] = temp;
+        return i+1; 
         //Compare elements and swap.
     } 
   
@@ -15,6 +49,30 @@ int partition(int arr[], int l, int h)
     void QuickSort(int arr[], int l, int h) 
     { 
         //Try using Stack Data Structure to remove recursion.
+        //create stack to store lower and upper bound of subarrays
+       Stack<Integer> stack = new Stack();
+
+       //push initial bound to the stack
+       stack.push(l);
+       stack.push(h);
+
+       //keep popping from stack while it is not empty
+       while(!stack.isEmpty()){
+            h = stack.pop();
+            l = stack.pop();
+        if(l < h){
+            int pivotIndex = partition(arr, l, h);
+            //If there are elements on left side of the pivot , push their indices to stack
+            if(pivotIndex-1>l){
+                stack.push(l);
+                stack.push(pivotIndex-1);
+            }
+            if(pivotIndex + 1 < h){
+                stack.push(pivotIndex+1);
+                stack.push(h);
+            }
+        }
+       }
     } 
   
     // A utility function to print contents of arr