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

题目链接/文章讲解：代码随想录

视频讲解：和组合问题有啥区别？回溯算法如何剪枝？| LeetCode：216.组合总和III_哔哩哔哩_bilibili

class Solution {
    List<List<Integer>> ans = new ArrayList<>();
    List<Integer> list = new ArrayList<>();
    public List<List<Integer>> combinationSum3(int k, int n) {
        backTracking(k, n, 1, 0);
        return ans;
    }

    public void backTracking(int k, int n, int startIndex, int sum) {
        if (sum > n) {
			return;
		}
        if (list.size() == k && sum == n) {
            ans.add(new ArrayList<>(list));
            return;
        }
        for (int i = startIndex; i <= 9; i++) {
            list.add(i);
            backTracking(k, n, i + 1, sum + i);
            list.removeLast();
        }
    }
}

?17.电话号码的字母组合?

题目链接/文章讲解：代码随想录

视频讲解：还得用回溯算法！| LeetCode：17.电话号码的字母组合_哔哩哔哩_bilibili

这道题特殊在是从两个集合中找，所以就没有startIndex了

class Solution {
    List<String> ans = new ArrayList<>();
    StringBuilder sb = new StringBuilder();
    public List<String> letterCombinations(String digits) {
        if (digits == null || digits.length() == 0) {
            return ans;
        }
        String[] num = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        backTraking(digits, num, 0);
        return ans;
    }

    public void backTraking(String digits, String num[], int index) {
        if (index == digits.length()) {
            ans.add(sb.toString());
            return;
        }
        String str = num[digits.charAt(index) - '0'];
        for (int i = 0; i < str.length(); i++) {
            sb.append(str.charAt(i));
            backTraking(digits, num, index + 1);
            sb.deleteCharAt(sb.length() - 1);
        }
    }
}