[LeetCode] 1286. Iterator for Combination 字母组合迭代器


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
  • All the characters of characters are unique.
  • At most 104 calls will be made to next and hasNext.
  • It is guaranteed that all calls of the function next are valid.

这道题让设计一个 CombinationIterator 类,初始化时给一个有序的字符串,然后一个组合的长度,有两个成员函数,一个是 next() 函数,是返回下一个按字母顺序,且是给定长度的组合,另一个 hasNext() 函数用来判断是否还存在下一个组合。给的例子可以很好的帮助我们理解题意,比如给的字符串是 "abc",返回组合的长度是2的话,则就是按照字母顺序来返回组合,"ab", "ac", "bc"。其实本质就是找出给定长度的所有组合,就拿给的例子 "abc" 来说吧,找出两个字母的组合,就是在三个字母中任意选两个,如果用1表示选择该字母,0表示不选的话,就有如下种情况:

a b c
0 0 0 -> ""
0 0 1 -> c
0 1 0 -> b
0 1 1 -> bc
1 0 0 -> a
1 0 1 -> ac
1 1 0 -> ab
1 1 1 -> abc

而每种情况正好对应一个二进制数,有三个字母的话,子集总个数为 2^3 = 8 个,每一种可以用一个二进制 mask 来表示,目标就是找出长度为2的组合。至于字母顺序不用太担心,将所有找出的组合放到一个 TreeSet 中,利用其自动排序的功能就行了。所以 mask 从1遍历到 2^n(这里的n为3),然后遍历给定字符串的长度,去看 mask 对应位上是1还是0,为1的话就将对应位的字母加到t中,最后看t的长度是多少,若是2的话,就加到 TreeSet 中。等将所有的长度为2的组合就找出来加到 TreeSet 中了之后,next 和 hasNext 函数就非常好实现了,用一个 iterator 就行了,next 就是看迭代器是否到末尾了,是的话返回空,否则返回当前指向到元素,并且迭代器后移一位。hasNext 就看迭代器是否到末尾了就行了,参见代码如下:


解法一:

class CombinationIterator {
public:
    CombinationIterator(string characters, int combinationLength) {
        st = generateAll(characters, combinationLength);
        cur = begin(st);
    }    
    string next() {
        return cur == end(st) ? "" : *cur++;
    }    
    bool hasNext() {
        return cur != end(st);
    }

private:
    set<string> st;
    set<string>::iterator cur;
    
    set<string> generateAll(string str, int len) {
        set<string> res;
        int n = 1 << str.size();
        for (int mask = 1; mask < n; ++mask) {
            string t = "";
            for (int i = 0; i < str.size(); ++i) {
                if (mask & (1 << i)) t += str[i];
            }
            if (t.size() == len) res.insert(t);
        }
        return res;
    }
};

我们也可以用迭代的方法来找出所有的组合,写法能稍微简洁一些,递归函数 generateAll 需要多加两个参数,start 和 res,分别是当前遍历到的位置,而当前组成的字符串。在递归函数中,若 len 为0了,则表示找到所求长度的组合,将其加入 TreeSet 中,否则就从 start 遍历到 n-len,然后调用递归函数,此时代入 len-1,i+1,和 res+str[i] 作为参数即可,参见代码如下:


解法二:

class CombinationIterator {
public:
    CombinationIterator(string characters, int combinationLength) {
        generateAll(characters, combinationLength, 0, "");
        cur = begin(st);
    }    
    string next() {
        return cur == end(st) ? "" : *cur++;
    }    
    bool hasNext() {
        return cur != end(st);
    }

private:
    set<string> st;
    set<string>::iterator cur;
    
    void generateAll(string str, int len, int start, string res) {
        if (len == 0) {
            st.insert(res);
            return;
        }
        for (int i = start; i <= (int)str.size() - len; ++i) {
            generateAll(str, len - 1, i + 1, res + str[i]);
        }
    }
};

Github 同步地址:

https://github.com/grandyang/leetcode/issues/1286


参考资料:

https://leetcode.com/problems/iterator-for-combination/

https://leetcode.com/problems/iterator-for-combination/discuss/451322/(JAVA)-Generate-Combinations

https://leetcode.com/problems/iterator-for-combination/discuss/451368/C%2B%2B-solution-with-multiple-pointers

https://leetcode.com/problems/iterator-for-combination/discuss/789164/C%2B%2B-Using-Bit-manipulation-or-Detail-Explain


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posted @ 2022-03-07 13:54  Grandyang  阅读(350)  评论(0编辑  收藏  举报
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