LeetCode in Racket

131. Palindrome Partitioning

Medium

Given a string s, partition s such that every substring of the partition is a palindrome. Return all possible palindrome partitioning of s.

A palindrome string is a string that reads the same backward as forward.

Example 1:

Input: s = “aab”

Output: [[“a”,”a”,”b”],[“aa”,”b”]]

Example 2:

Input: s = “a”

Output: [[“a”]]

Constraints:

• 1 <= s.length <= 16
• s contains only lowercase English letters.

Solution

(define/contract (partition s)
  (-> string? (listof (listof string?)))
  (define res '())
  (define (backtracking currArr start)
    (if (= start (string-length s))
        (set! res (cons (reverse currArr) res))
        (for ([end (in-range start (string-length s))])
          (when (is-palindrome? s start end)
            (backtracking (cons (substring s start (add1 end)) currArr) (add1 end))))))
  (backtracking '() 0)
  res)

(define (is-palindrome? s start end)
  (let loop ((i start) (j end))
    (or (>= i j)
        (and (char=? (string-ref s i) (string-ref s j))
             (loop (add1 i) (sub1 j))))))

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