Merge pull request #30 from kartikeytripathi/master

Add Binary Search in C++

Author: Bhrigu Kansra

contributors.md

@@ -1,18 +1,11 @@
 # List of all contributors
 
-
-1.  [Bhrigu Kansra](https://github.com/kinetickansra)
-2.  [Tathya Thaker](https://github.com/thetathya)
-3.  [Aslan](https://github.com/impeccableaslan)
-4.  [Naman Manchanda](https://github.com/namanmanchanda09)
-5.  [Harsh Arora](https://github.com/aroraharsh010)
-6.  [Ujjval Patel](https://github.com/Ujjval-Patel)
-7.  [Shaguna Awasthi](https://github.com/Shagunaawasthi)
-8.  [Joy Banerjee](https://github.com/joybanerjee08)
-9.  [Ambika](https://github.com/Ambika55)
-10. [Shivam Yadav](https://github.com/shivamyadav2512)
-11. [Nikhil Arora](https://github.com/nikhilarora06
-
-12. [Your Name](https://github.com/yourprofile)
-
-
+1. [Bhrigu Kansra](https://github.com/kinetickansra)
+2. [Tathya Thaker](https://github.com/thetathya)
+3. [Aslan](https://github.com/impeccableaslan)
+4. [Naman Manchanda](https://github.com/namanmanchanda09)
+5. [Harsh Arora](https://github.com/aroraharsh010)
+6. [Ujjval Patel](https://github.com/Ujjval-Patel)
+7. [Shaguna Awasthi](https://github.com/Shagunaawasthi)
+8. [Joy Banerjee](https://github.com/joybanerjee08)
+9. [Kartikey Tripathi](https://github.com/kartikeytripathi)

searching/binary_search.cpp

@@ -0,0 +1,62 @@
+/*
+    Binary Search
+    -----------
+    A searching algorithm that finds the position of a target value
+    within a sorted array.
+    
+    Time complexity
+    ---------------
+    O(log(N)), where N is the number of elements in the array.
+   
+   Space complexity
+    ----------------
+    O(1).
+*/
+
+#include <iostream>
+#include <vector>
+
+using namespace std;
+
+int binarySearch(const int value, const vector<int>& sortedVect, const int low, const int high) {
+    int mid = (low + high) / 2;
+
+    if (value == sortedVect[mid])
+        return mid;
+    else if (low <= high) {
+        if (value < sortedVect[mid]) {
+            // value must be between indices low and mid-1, if exists
+            return binarySearch(value, sortedVect, low, mid-1);
+        }
+        else if (value > sortedVect[mid]) {
+            // value must be between indices mid-1 and high, if exists
+            return binarySearch(value, sortedVect, mid+1, high);
+        }
+    }
+
+    return -1;
+}
+
+int main() {
+    int value;
+    cout << "Enter the value to search for : ";
+    cin >> value;
+
+    int size;
+    cout << "Enter the input size : ";
+    cin >> size;
+
+    vector<int> inputVect(size);    // supposedly sorted values
+    cout << "Enter " << size << " integers in ascending order :\n";
+    for (int& val: inputVect)
+        cin >> val;
+
+    int index = binarySearch(value, inputVect, 0, size-1);
+    cout << "\n";
+    if (index != -1)
+        cout << "Found " << value << " at position " << (index + 1) << "\n";
+    else
+        cout << "Either " << value << " is not present among the input values, or the values aren\'t sorted\n";
+
+    return 0;
+}

searching/ternary_search.cpp

@@ -0,0 +1,104 @@
+/*
+    Ternary search
+    --------------
+    A searching algorithm that finds the position of a maximum(minimum) value
+    within an unimodal array.
+    
+    Time complexity
+    ---------------
+    O(log(N)), where N is the number of elements in the array.
+    
+    Space complexity
+    ----------------
+    O(1).
+*/
+
+#include <iostream>
+#include <vector>
+
+using namespace std;
+
+enum Pattern {
+    ASCEND_THEN_DESCEND,
+    DESCEND_THEN_ASCEND
+};
+
+/*
+    ternary_search
+    -------------
+    If the values first ascend and then descend, this function finds the position
+    of the maximum value. Otherwise, if they first descend and then ascend, it
+    finds the position of the minimum value.
+*/
+template <typename T>
+size_t ternary_search(const vector<T>& values, const Pattern& pattern) {
+    // left and right are the edges of the interval of search
+    size_t left = 0;
+    size_t right = values.size() - 1;
+
+    bool changed = true;
+    size_t mid1, mid2;
+    while (right - left > 1 and changed) {  // if the interval is not shrinking,
+                                            // its size already equals O(1)
+        changed = false;
+        mid1 = left + (right - left) / 3;
+        mid2 = right - (right - left) / 3;
+        if ((pattern and values[mid1] < values[mid2])
+            or (!pattern and values[mid1] > values[mid2])) {
+            changed |= (right != mid2);
+            right = mid2;
+        }
+        else {
+            changed |= (left != mid1);
+            left = mid1;
+        }
+    }
+
+    T min_value = values[left];
+    T max_value = values[left];
+    size_t min_index = left;
+    size_t max_index = left;
+    for (size_t index = left + 1; index <= right; index++) {
+        if (min_value > values[index]) {
+            min_value = values[index];
+            min_index = index;
+        }
+        if (max_value < values[index]) {
+            max_value = values[index];
+            max_index = index;
+        }
+    }
+
+    return pattern == ASCEND_THEN_DESCEND ? max_index : min_index;
+}
+
+#ifndef TERNARY_SEARCH_TEST
+int main() {
+    int size;
+    cout << "Enter the input size : ";
+    cin >> size;
+
+    vector <int> input_values(size);    //supposedly unimodal array
+    cout << "Enter " << size << " values:\n";
+    for (int& value: input_values)
+        cin >> value;
+
+    string answer;
+    cout << "\nDo the values first ascend then descend?"
+         << "If otherwise, enter \"no\".\n"
+         << "[Y]es / [n]o : ";
+    getline(cin, answer);
+
+    Pattern pattern = ASCEND_THEN_DESCEND;  // answer is "yes" by default
+    if (answer[0] == 'n' or answer[0] == 'N')   // user answers "no"
+        pattern = Pattern::DESCEND_THEN_ASCEND;
+
+    int index = ternary_search(input_values, pattern);
+    if (pattern == ASCEND_THEN_DESCEND)
+        cout << "Maximum value has position " << index << "\n";
+    else
+        cout << "Minimum value has position " << index << "\n";
+
+    return 0;
+}
+#endif