CPP/strings: Add shift_cipher solution (shift cipher problem)

Author: Mojirade18

CPP/strings/shift_cipher.cpp

@@ -0,0 +1,93 @@
+/*
+Problem Name: Shift Cipher
+Language: C++
+Description:
+    Given N test cases. For each test case:
+    - Read an integer k (the shift value).
+    - Read a string t (the text).
+    - If the word "the" appears as a separate word in t, shift letters backward by k.
+    - Otherwise, shift letters forward by k.
+    - Spaces and non-alphabetic characters remain unchanged.
+
+Approach:
+    - Iterate over each character.
+    - Check if it is an alphabet; shift it accordingly using ASCII values.
+    - Use modulo 26 arithmetic for wrap-around.
+    - Before shifting, determine if the word "the" exists in the string.
+
+Example Input:
+2
+3
+hello world
+1
+the cat is here
+
+Example Output:
+khoor zruog
+sfd bzs hr ifsf
+
+Time Complexity: O(L) per test case (L = length of string)
+Space Complexity: O(L)
+*/
+
+#include <bits/stdc++.h>
+using namespace std;
+
+// Function to perform the shift cipher
+string shiftCipher(const string &text, int k, int direction) {
+    string result;
+    result.reserve(text.size());
+
+    for (char c : text) {
+        if (c == ' ') {
+            result += ' ';
+        } else if (isalpha(static_cast<unsigned char>(c))) {
+            char base = isupper(static_cast<unsigned char>(c)) ? 'A' : 'a';
+            int shifted = ((c - base) + direction * k) % 26;
+            if (shifted < 0) shifted += 26;  // handle negative modulo
+            result += static_cast<char>(shifted + base);
+        } else {
+            result += c;
+        }
+    }
+
+    return result;
+}
+
+int main() {
+    ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    int N;
+    cin >> N;
+    cin.ignore(); // consume newline
+
+    for (int i = 0; i < N; ++i) {
+        int k;
+        cin >> k;
+        cin.ignore();
+
+        string t;
+        getline(cin, t);
+
+        // Check if "the" appears as a word (case-insensitive)
+        bool containsThe = false;
+        string word;
+        for (size_t j = 0; j <= t.size(); ++j) {
+            if (j == t.size() || t[j] == ' ') {
+                if (!word.empty() && word == "the") {
+                    containsThe = true;
+                    break;
+                }
+                word.clear();
+            } else {
+                word += tolower(static_cast<unsigned char>(t[j]));
+            }
+        }
+
+        // Direction: -1 if "the" found, +1 otherwise
+        cout << shiftCipher(t, k, containsThe ? -1 : 1) << '\n';
+    }
+
+    return 0;
+}