Day 41 next permutation of string

Author: mandliya

README.md

@@ -4,9 +4,9 @@
 
 | Current Status|     Stats     |
 | :------------: | :----------: |
-| Total Problems | 59 |
-| Current Streak | 40 |
-| Longest Streak | 40 ( August 17, 2015 - September 25, 2015 ) |
+| Total Problems | 60 |
+| Current Streak | 41 |
+| Longest Streak | 41 ( August 17, 2015 - September 26, 2015 ) |
 
 </center>
 
@@ -92,6 +92,7 @@ Include contains single header implementation of data structures and some algori
 | Problem | Solution |
 | :------------ | :----------: |
 | Implementation of Robin-Karp algorithm for string search | [robinKarpStringMatching.cpp](string_problmes/robinKarpStringMatching.cpp) |
+| Find next permutation of a given string, ie. rearrange the given string sucht a way that is next lexicographically greater string than given string | [next_permutation.cpp](string_problems/next_permutations.cpp)|
 
 ### Math Problems
 | Problem | Solution |

string_problems/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;
+}