solved on 12/12/23

Author: Priyanshu-Ganatra

CS_34_ReverseFirstK_elements.cpp

@@ -0,0 +1,50 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+queue<int> reverseElements(queue<int> q, int k)
+{
+    stack<int> s;
+    int n = q.size();
+    // push first k element in stack
+    for (int i = 0; i < k; i++)
+    {
+        s.push(q.front());
+        q.pop();
+    }
+
+    // push all k elements in queue
+    while (!s.empty())
+    {
+        q.push(s.top());
+        s.pop();
+    }
+
+    // pop and push first n-k elements into queue again
+    for (int i = 0; i < n - k; i++)
+    {
+        q.push(q.front());
+        q.pop();
+    }
+    return q;
+}
+
+int main()
+{
+    queue<int> q;
+    q.push(10);
+    q.push(20);
+    q.push(30);
+    q.push(40);
+    q.push(50);
+
+    int k = 3;
+    q = reverseElements(q, k);
+
+    cout << "Queue after reversing first " << k << " elements: ";
+    while (!q.empty())
+    {
+        cout << q.front() << " ";
+        q.pop();
+    }
+    return 0;
+}

CS_35_InterleaveTheFirstHalfOfTheQueueWithTheSecondHalf.cpp

@@ -0,0 +1,49 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+void interLeaveQueue(queue<int> &first)
+{
+    queue<int> second;
+    // push 1st half of first queue in 2nd queue
+    int size = first.size();
+    for (int i = 0; i < size / 2; i++)
+    {
+        second.push(first.front());
+        first.pop();
+    }
+
+    // merge both halves into the og queue named as first
+    for (int i = 0; i < size / 2; i++)
+    { 
+        first.push(second.front());
+        second.pop();
+
+        first.push(first.front());
+        first.pop();
+    }
+}
+
+int main()
+{
+    queue<int> q;
+    q.push(11);
+    q.push(12);
+    q.push(13);
+    q.push(14);
+    q.push(15);
+    q.push(16);
+    q.push(17);
+    q.push(18);
+    q.push(19);
+    q.push(20);
+
+    interLeaveQueue(q);
+
+    while (!q.empty())
+    {
+        cout << q.front() << " ";
+        q.pop();
+    }
+    cout << endl;
+    return 0;
+}

CS_36_FirstNegativeInAWindowOfSizeK.cpp

@@ -0,0 +1,51 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+// tc
+vector<int> firstNegativeInteger(vector<int> &arr, int k, int n)
+{
+    queue<int> q; // store the index of the negative elements in the window
+    vector<int> ans; // store the answer elements
+    // process first k elements - 1st window
+    for (int i = 0; i < k; i++)
+    {
+        if (arr[i] < 0) 
+            q.push(i);
+    }
+
+    // process remaining windows -> remove the element which is out of the window and add the new e lement
+    for (int i = k; i < n; i++)
+    {
+        // before further processing, add the negative element of the previous window in the answer arra if it exists in the queue
+        if (!q.empty())
+            ans.push_back(arr[q.front()]);
+        else
+            ans.push_back(0);
+        // remove the front element if it is out of the window
+        if (i - q.front() >= k)
+            q.pop();
+        // add the new element if it is negative, i.e. consider it for the next window if is negative
+        if (arr[i] < 0)
+            q.push(i);
+    }
+
+    // process the last window
+    if (!q.empty())
+        ans.push_back(arr[q.front()]);
+    else
+        ans.push_back(0);
+
+    return ans;
+}
+
+int main()
+{
+    vector<int> arr = {2, -5, 4, -1, -2, 0, 5};
+    int k = 3;
+    int n = arr.size();
+    vector<int> ans = firstNegativeInteger(arr, k, n);
+    for (auto x : ans)
+        cout << x << " ";
+    cout << endl;
+    return 0;
+}

CS_37_FirstNonRepeatingCharInAStream.cpp

@@ -0,0 +1,50 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+vector<char> firstNonRepeating(string stream)
+{
+    vector<char> ans;
+    queue<char> q;
+    int freq[26] = {0};
+
+    // traverse input stream, increase freq count of current char, add it to queue
+    // remove it from queue if it is repeating (freq more than 1), this will help
+    // to constrain the comparisons as we remove the repeating char, we need not to
+    // worry about it in the next iteration.
+    // if the current char isn't repeating add it to ans and stop
+    for (int i = 0; i < stream.length(); i++)
+    {
+        char ch = stream[i];
+        freq[ch - 'a']++;
+        q.push(ch);
+
+        // finding answer
+        while (!q.empty())
+        {
+            char frontChar = q.front();
+            if (freq[frontChar - 'a'] > 1)
+            {
+                q.pop();
+            }
+            else
+            {
+                // freq = 1, this is the answer
+                ans.push_back(frontChar);
+                break;
+            }
+        }
+        // here either the entire while loop has exhausted indicating that all chars
+        // were repeated, or the 1st non-repeating char was encountered and loop broke
+    }
+    return ans;
+}
+
+int main()
+{
+    string stream = "bbabcbcab ";
+    vector<char> ans = firstNonRepeating(stream);
+    for (auto ch : ans)
+        cout << ch << " ";
+    cout << endl;
+    return 0;
+}

Queue/ReverseQueue.cpp

@@ -0,0 +1,40 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+// tc: O(n) sc: O(n) - iterative
+// void reverseQueue(queue<int> &q){
+    // stack<int> s;
+    // while(!q.empty()){
+    //     s.push(q.front());
+    //     q.pop();
+    // }
+    // while(!s.empty()){
+    //     q.push(s.top());
+    //     s.pop();
+    // }
+// } 
+
+// tc: O(n) sc: O(n) - recursive
+void reverseQueue(queue<int> &q){
+    if(q.empty()) return; // base case
+    int x = q.front();
+    q.pop();
+    reverseQueue(q);
+    q.push(x);
+}
+
+int main(){
+    queue<int> q;
+    q.push(10);
+    q.push(20);
+    q.push(30);
+    q.push(40);
+    q.push(50);
+    reverseQueue(q);
+    cout << "Queue after reversing: ";
+    while(!q.empty()){
+        cout << q.front() << " ";
+        q.pop();
+    }
+    return 0;
+}