LeetCode in Racket

39. Combination Sum

Medium

Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where the chosen numbers sum to target. You may return the combinations in any order.

The same number may be chosen from candidates an unlimited number of times. Two combinations are unique if the frequency of at least one of the chosen numbers is different.

It is guaranteed that the number of unique combinations that sum up to target is less than 150 combinations for the given input.

Example 1:

Input: candidates = [2,3,6,7], target = 7

Output: [[2,2,3],[7]]

Explanation: 2 and 3 are candidates, and 2 + 2 + 3 = 7. Note that 2 can be used multiple times.

7 is a candidate, and 7 = 7.

These are the only two combinations.

Example 2:

Input: candidates = [2,3,5], target = 8

Output: [[2,2,2,2],[2,3,3],[3,5]]

Example 3:

Input: candidates = [2], target = 1

Output: []

Constraints:

• 1 <= candidates.length <= 30
• 1 <= candidates[i] <= 200
• All elements of candidates are distinct.
• 1 <= target <= 500

Solution

(define (create-task-rec elems target next-list result)
  (if (= 0 (length next-list)) result
      (if (< target (+ (foldl + 0 elems) (car next-list)))
          (create-task-rec elems target (cdr next-list) result)
          (create-task-rec elems target (cdr next-list)
                           (append result
                                   (list (list (append elems (list (car next-list)))
                                               next-list)))))))

(define (create-task elems target next-list)
  (create-task-rec elems target next-list (list)))

(define (dfs result task target)
  (if (= 0 (length task)) result
      (let* ([current-task (car task)]
             [new-task (create-task (car current-task)
                                     target
                                     (cadr current-task))]
             [result (if (= (foldl + 0 (car current-task)) target)
                         (append result (list (car current-task)))
                         result)]
             [task (append new-task (cdr task))])
        (dfs result task target))))

(define (start-dfs lst target) (dfs (list) (create-task (list) target lst) target))

(define/contract (combination-sum candidates target)
  (-> (listof exact-integer?) exact-integer? (listof (listof exact-integer?)))
　(start-dfs candidates target))

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