Added Next_permutation.cpp

Contributors.md

@@ -14,4 +14,4 @@
 12. [Hacker-set](https://github.com/Hacker-set)]
 13. [Prateek Sengar]https://github.com/prtksengar3)
 14. [ANant Rungta](https://github.com/Anant016)
-
+15. [Pratyay Banerjee](https://github.com/Neilblaze)

Next_permutation.cpp

@@ -0,0 +1,54 @@
+/**
+ *  Implement next permutation algorithm. i.e
+ *  Given a word w, rearrange the letters of w to construct another word s in such a way that
+ *  s is lexicographically greater than w.
+ *  In case of multiple possible answers, find the lexicographically smallest one.
+ *  Example:
+ *  ab --> ba
+ *  bb --> bb
+ *  hefg --> hegf
+ *  dhck --> dhkc
+ *  dkhc --> hcdk
+ */
+
+#include <iostream>
+
+std::string
+next_permutation( std::string str)
+{
+  int len = str.length();
+  int i = len -1;
+  // We will iterate string from end, and once we have encountered a pair of letters
+  // such that str[i] < str[i-1] , i-1 would become our pivot
+  while( i > 0 && str[i] <= str[i-1] ) {
+    --i;
+  }
+  if ( i == 0 ) {
+    return str;
+  }
+  //our pivot right now would be str[i-1]
+  int j = len - 1;
+  while( j >= i && str[j] <= str[i-1] ) {
+    --j;
+  }
+  std::swap(str[i-1], str[j]);
+  j = len - 1;
+  //reverse the suffix
+  while( i < j ) {
+    std::swap(str[i], str[j]);
+    --j;
+    ++i;
+  }
+  return str;
+}
+
+int main()
+{
+  std::string str, str_next;
+  std::cout << "Enter a string : ";
+  std::cin >> str;
+  str_next = next_permutation(str);
+  std::cout << "Next permutation of " << str << " is "
+            << str_next << std::endl;
+  return 0;
+}