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Programming Praxis 19 Aug 2011 (first non-repeating char) solution in PLT Racket
#lang racket
(define (first-uniq chars)
(when (empty? chars)
(raise "There are no unique letters"))
(let ([c (first chars)])
(if (memq c (rest chars)) ; MEMQ is like MEMBER with EQ?
(first-uniq (remq* (list c) chars)) ; N.B the tail-recursion here
c)))
(require rackunit)
(check-equal? (first-uniq (string->list "aabcbcdeef"))
#\d)
(check-equal? (first-uniq (string->list "faabcbcdeef"))
#\d)
(check-equal? (first-uniq (string->list "aabcbcdeefd"))
#\f)
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