added pandit's algorithm

Author: vidhanjhawar

Permutations/next_greatest_permutation.py

@@ -0,0 +1,44 @@
+# -*- coding: utf-8 -*-
+"""
+Created on Tue Oct 15 18:11:37 2019
+
+@author: Vidhan Jhawar
+"""
+
+'''
+Narayana Pandit's algorithm to find next lexicographically greatest permutation
+
+Algorithm:
+1. Find the largest index k such that a[k] < a[k + 1]. If no such index exists, the permutation is the last permutation.
+2. Find the largest index l greater than k such that a[k] < a[l].
+3. Swap the value of a[k] with that of a[l].
+4. Reverse the sequence from a[k + 1] up to and including the final element a[n].
+'''
+
+import math
+
+def nextGreater(w):
+    w=list(w)
+    l=len(w)
+    flag=1
+    for i in range(l-2,-1,-1):
+        if(w[i]<w[i+1]):
+            flag=0
+            break
+    if(flag==1):
+        return 'no answer'
+    for j in range(l-1,i,-1):
+        if(w[j]>w[i]):
+            break
+    temp=w[i]
+    w[i]=w[j]
+    w[j]=temp
+    for k in range(i+1,math.ceil((l+i-1)/2)+1):
+        temp=w[k]
+        w[k]=w[l-k+i]
+        w[l-k+i]=temp
+    return ''.join(w)
+
+if __name__ == '__main__':
+    w=input()
+    print(nextGreater(w))