Day 25 | 回溯 216.组合总和III 、17.电话号码的字母组合

216.组合总和III

题目
文章讲解
视频讲解

思路：
以回溯函数做对称，上面做了什么操作，下面也是

class Solution {
    List<List<Integer>> result = new ArrayList<>();
    LinkedList<Integer> path = new LinkedList<>();

    public List<List<Integer>> combinationSum3(int k, int n) {
        travelsal(k, n, 1, 0);
        return result;
    }

    private void travelsal(int k, int n, int startIndex, int sum) {
        if (sum == n && path.size() == k) {
            result.add(new ArrayList<>(path));
            return;
        }
        for (int i = startIndex; i <= 9 - (k - path.size()) + 1; i++) {
            if (sum > n || path.size() > k)
                break;
            path.add(i);
            sum += i;
            travelsal(k, n, i + 1, sum);
            sum -= i;
            path.removeLast();
        }
    }
}

17.电话号码的字母组合

题目
文章讲解
视频讲解

思路：思路上和组合很像，不同的是对字符串进行操作

class Solution {
    List<String> list = new ArrayList<>();

    public List<String> letterCombinations(String digits) {
        if (digits == null || digits.length() == 0)
            return list;
        String[] numString = { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };
        travelsal(digits, numString, 0);
        return list;
    }

    StringBuilder temp = new StringBuilder();

    public void travelsal(String digits, String[] numString, int num) {
        if (num == digits.length()) {
            list.add(temp.toString());
            return;
        }

        String str = numString[digits.charAt(num) - '0'];

        for (int i = 0; i < str.length(); i++) {
            temp.append(str.charAt(i));
            travelsal(digits, numString, num + 1);
            temp.deleteCharAt(temp.length() - 1);
        }
    }
}

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