chore(CPlusPlus): add longest common prefix and first unique character (#619)

algorithms/CPlusPlus/README.md

@@ -115,6 +115,8 @@
 - [Print first letter of every word](Strings/print-first-letter.cpp)
 - [Display longest name in a string array](Strings/longest-name.cpp)
 - [Boyer Moore pattern search](Strings/Boyer_Moore.cpp)
+- [Longest common prefix](Strings/longest-common-prefix.cpp)
+- [First unique character in the string](Strings/first-unique-character.cpp)
 
 ## Trees
 

algorithms/CPlusPlus/Strings/first-unique-character.cpp

@@ -0,0 +1,61 @@
+/*
+Description: Given a string 's', return the index of the first unique character from the string. If not found, return -1
+
+Time Complexity: O(n) where n is the length of the string
+*/
+#include <iostream>
+#include <map>
+using namespace std;
+
+//function starts
+int firstUniqChar(string s)
+{
+    //declare a map to store the frequency of the character
+    map<char, int> m;
+    //storing the character and its frequency in the map for every character in the string
+    for (char c : s)
+    {
+        m[c]++;
+    }
+    //the frequency of the non-repreating or unique character will be 1
+    //1 indicates that it had appeared only once in the string
+    //since we want to search from the beginning of the string
+    //use for loop from beginning of the string
+    //to check if that character appeared for a single time
+    //if yes, return the index
+    for (int i = 0; i < s.size(); i++)
+    {
+        if (m[s[i]] == 1)
+        {
+            return i;
+        }
+    }
+    return -1;
+}
+
+//main starts
+int main()
+{
+    string s;
+    cout << "Enter a string: \n";
+    getline(cin, s);
+    cout << firstUniqChar(s);
+    return 0;
+}
+
+/*
+Example 1:
+
+Input: s = "leetcode"
+Output: 0
+
+Example 2:
+
+Input: s = "loveleetcode"
+Output: 2
+
+Example 3:
+
+Input: s = "aabb"
+Output: -1
+*/

algorithms/CPlusPlus/Strings/longest-common-prefix.cpp

@@ -0,0 +1,62 @@
+/*
+Description: Given two strings, return the longest common prefix from both of the strings.
+Longest common prefix means returning the starting characters which are common in both the strings.
+For e.g., 
+s1 = "hello"
+s2 ="heat"
+Longest common prefix will be "he" 
+Because only two characters from the starting are common between them.
+
+Time Complexity: O(n) where n is the length of the string
+*/
+#include <iostream>
+using namespace std;
+
+//function starts
+string lcp(string s1, string s2)
+{
+    string ans = "";
+    if (s1.size() == 0 || s2.size() == 0)
+    {
+        return ans;
+    }
+    //in 'm' variable, we are storing the smallest string's size from s1 and s2
+    //because the prefix will always be at most the size of the smallest string
+    int m = s1.size() < s2.size() ? s1.size() : s2.size();
+    for (int i = 0; i < m; i++)
+    {
+        //if both the starting characters are not same, then break from the for loop
+        if (s1[i] != s2[i])
+        {
+            break;
+        }
+        //else if they are same, add the character in the answer string
+        else
+        {
+            ans += s1[i];
+        }
+    }
+    return ans;
+}
+
+//main starts
+int main()
+{
+    string s1;
+    string s2;
+    cout << "Enter string-1 : \n";
+    getline(cin, s1);
+    cout << "Enter string-2: \n";
+    getline(cin, s2);
+    cout << "Longest common prefix is: " << lcp(s1, s2);
+    return 0;
+}
+
+/*
+Sample Input:
+ string s1 = "flower"
+string s2 = "flight"
+
+Output
+Longest common prefix is: fl
+*/