added queue using stack

Author: shiningflash

queue_using_1_stack.cpp

@@ -0,0 +1,43 @@
+#include <iostream>
+#include <stack>
+using namespace std;
+
+stack <int> st;
+
+void enqueue() {
+    int val;
+    cin >> val;
+    st.push(val);
+}
+
+int dequeue() {
+    if (st.size() == 1) {
+        int val = st.top();
+        st.pop();
+        return val;
+    }
+    int curValue = st.top();
+    st.pop();
+    int rec = dequeue();
+    st.push(curValue);
+    return rec;
+}
+
+int main() {
+    string cmd;
+    for (int i = 0; i < 10; i++) {
+        cin >> cmd;
+        if (tolower(cmd[0]) == 'e') { // cmd == "enqueue"
+            enqueue();
+        }
+        else if (tolower(cmd[0]) == 'd') { // cmd == 'dequeue'
+            if (st.empty()) {
+                printf("Error: queue is empty\n");
+                exit(EXIT_FAILURE);
+            }
+            int val = dequeue();
+            printf("dequeued: %d\n", val);
+        }
+    }
+    return 0;
+}

queue_using_stack.cpp

@@ -0,0 +1,59 @@
+/*
+    it takes two stacks to perform the task,
+    but it's time efficient, only in Linear time
+
+    There are also another approach, which is
+    not time efficient, but use only one stack
+*/
+
+#include <iostream>
+#include <stack>
+using namespace std;
+
+/*
+| 3 |  | 1 |
+| 2 |  | 2 |
+| 1 |  | 3 |
+-----  -----
+ enq    deq
+*/
+
+stack<int> enq, deq;
+
+void enqueue() {
+    int val;
+    cin >> val;
+    enq.push(val);
+}
+
+void dequeue() {
+    if (deq.empty()) { // if deq empty, transfer all enq value to deq in reverse order
+        int val;
+        while (!enq.empty()) {
+            int val = enq.top();
+            enq.pop();
+            deq.push(val);
+        }
+    }
+    if (deq.empty()) { //  if still empty
+        printf("Error: queue is empty\n");
+        exit(EXIT_FAILURE);
+    }
+    int val = deq.top();
+    deq.pop();
+    printf("dequeued %d\n", val);
+}
+
+int main() {
+    string cmd;
+    for (int i = 0; i < 10; i++) {
+        cin >> cmd;
+        if (tolower(cmd[0]) == 'e') { // cmd == "enqueue"
+            enqueue();
+        }
+        else if (tolower(cmd[0]) == 'd') { // cmd == 'dequeue'
+            dequeue();
+        }
+    }
+    return 0;
+}