calculate occurrences of first character upto ith index from left
for i in range ( 1 , n ) : NEW_LINE INDENT if ( str1 [ i ] == substr [ 0 ] ) : NEW_LINE INDENT preLeft [ i ] = preLeft [ i - 1 ] + 1 NEW_LINE DEDENT else : NEW_LINE INDENT preLeft [ i ] = preLeft [ i - 1 ] NEW_LINE DEDENT DEDENT if ( str1 [ n - 1 ] == substr [ 2 ] ) : NEW_LINE INDENT preRight [ n - 1 ] += 1 NEW_LINE DEDENT