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Design the CombinationITerator
class:
CombinationIterator(string characters, int combinationLength)
Initializes the object with a string characters
of sorted distinct lowercase English letters and a number combinationLength
as arguments.next()
Returns the next combination of length combinationLength
in lexicographical order.hasNext()
Returns true
if and only if there exists a next combination.Example 1:
Input ["CombinationIterator", "next", "hasNext", "next", "hasNext", "next", "hasNext"] [["abc", 2], [], [], [], [], [], []] Output [null, "ab", true, "ac", true, "bc", false] Explanation CombinationIterator itr = new CombinationIterator("abc", 2); itr.next(); // return "ab" itr.hasNext(); // return True itr.next(); // return "ac" itr.hasNext(); // return True itr.next(); // return "bc" itr.hasNext(); // return False
Constraints:
1 <= combinationLength <= characters.length <= 15
characters
are unique.104
calls will be made to next
and hasNext
.next
are valid.字母组合迭代器。
请你设计一个迭代器类,包括以下内容:
一个构造函数,输入参数包括:一个 有序且字符唯一 的字符串 characters(该字符串只包含小写英文字母)和一个数字 combinationLength 。函数 next() ,按 字典序 返回长度为 combinationLength 的下一个字母组合。函数 hasNext() ,只有存在长度为 combinationLength 的下一个字母组合时,才返回 True;否则,返回 False。
来源:力扣(LeetCode)链接:https://leetcode-cn.COM/PRoblems/iterator-for-combination著作权归领扣网络所有。商业转载请联系官方授权,非商业转载请注明出处。
这是一道回溯的题目,如果不熟悉可以先做77题。77题的组合问题是给你从 1 到 N 的数字请你列出所有长度为 K 的数字的组合,这道题无非是把数字换成了字母。
时间O(n)
空间O(n)
Java实现
1 class CombinationIterator { 2 Queue<String> queue; 3 4 public CombinationIterator(String characters, int combinationLength) { 5 queue = new LinkedList<>(); 6 helPEr(characters, 0, "", combinationLength, queue); 7 } 8 9 public void helper(String characters, int start, String soFar, int k, Queue<String> queue) { 10 // k = how may characters left for current string, k = combinationLength - soFar.length() 11 if (k == 0) { 12 queue.offer(soFar); 13 return; 14 } 15 for (int i = start; i < characters.length(); i++) { 16 helper(characters, i + 1, soFar + characters.charAt(i), k - 1, queue); 17 } 18 } 19 20 public String next() { 21 return queue.poll(); 22 } 23 24 public boolean hasNext() { 25 return !queue.iSEMpty(); 26 } 27 } 28 29 /** 30 * Your CombinationIterator object will be instantiated and called as such: 31 * CombinationIterator obj = new CombinationIterator(characters, combinationLength); 32 * String param_1 = obj.next(); 33 * boolean param_2 = obj.hasNext(); 34 */
相关题目
77. Combinations
1286. Iterator for Combination
LeetCode 题目总结
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