代码随想录算法训练营第二十二天-216组合总和III、17电话号码的字母组合

前言
状态：216 可以 AC。17 看了部分解析写出来的。
更新

1	24-06-10 初始记录

初步题解

216 组合总和 III

题目链接：(https://leetcode.cn/problems/combination-sum-iii)

public class LE22 {  
    public static void main(String[] args) {  
        List<List<Integer>> combinationSum3 = combinationSum3(9, 45);  
        for (List<Integer> integerList : combinationSum3) {  
            System.out.println(Arrays.toString(integerList.stream().mapToInt(n -> n).toArray()));  
        }  
    }  
  
    public static List<List<Integer>> combinationSum3(int k, int n) {  
        // 结果集  
        List<List<Integer>> result = new ArrayList<>();  
        // 单个结果  
        List<Integer> list = new ArrayList<>();  
  
        combinationSum(1, k, n, 0, list, result);  
        return result;  
    }  
  
    private static void combinationSum(int start, int k, int n, int sum, List<Integer> list, List<List<Integer>> result) {  
        // 剪枝操作  
        if (sum > n) {  
            return;  
        }  
  
        if (list.size() == k) {  
            if (sum == n) {  
                List<Integer> resultOne = new ArrayList<>(list);  
                result.add(resultOne);  
            }  
            return;  
        }  
  
        for (int j = start; j <= 9 - (k - list.size()) + 1; j++) {  
            list.add(j);  
            combinationSum(j + 1, k, n, sum + j, list, result);  
            list.remove(list.size() - 1);  
        }  
    }  
}

17 电话号码的字母组合

题目链接：(https://leetcode.cn/problems/letter-combinations-of-a-phone-number)

/**  
 * @param digits 按键数组  
 * @return 结果  
 */  
public static List<String> letterCombinations(String digits) {  
    List<String> list = new ArrayList<>();  
    String[] buttonList = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};  
  
    if (digits == null || "".equals(digits)) {  
        return list;  
    }  
  
    combinations(0, digits, buttonList, "", list);  
    return list;  
}  
  
/**  
 * * @param i 遍历的层数  
 */  
private static void combinations(int i, String digits, String[] buttonList, String str, List<String> list) {  
    if (i == digits.length()) {  
        list.add(str);  
        return;  
    }  
  
    int digit = Integer.parseInt(digits.charAt(i) + "");  
    String letter = buttonList[digit];  
    for (int j = 0; j < letter.length(); j++) {  
        str+=letter.charAt(j);  
        combinations(i + 1, digits, buttonList, str, list);  
        str = str.substring(0, str.length() - 1);  
    }  
}

看解析

216 组合总和 III

题目链接/文章讲解：(https://programmercarl.com/0216.组合总和III.html)

视频讲解：https://www.bilibili.com/video/BV1wg411873x

17 电话号码的字母组合

题目链接/文章讲解：(https://programmercarl.com/0017.电话号码的字母组合.html)

视频讲解：https://www.bilibili.com/video/BV1yV4y1V7Ug