代码随想录 第25天 | ● 216.组合总和III ● 17.电话号码的字母组合

leetcode：216. 组合总和 III - 力扣（LeetCode）

class Solution {
        List<List<Integer>> res = new ArrayList<>();
        LinkedList<Integer> link = new LinkedList<>();

    public List<List<Integer>> combinationSum3(int k, int n) {
        backtracking(k,n,0,1);
        return res;
    }
    public void backtracking(int k , int n , int sum , int strartIndex){
        if( sum > n) return;
        if(link.size() > k) return;
        //和等于目标值，存入二维数组
        if (link.size() == k){
            if(sum == n){
                res.add(new ArrayList<>(link));
                return;
            }

        }
        for(int i = strartIndex;i <= 9; i++){
            link.add(i);
            sum += i;
            backtracking(k,n,sum,i+1);
            sum -= i;
            link.removeLast();
        }

    }
}

 leetcode：17. 电话号码的字母组合 - 力扣（LeetCode）

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"};
        backtracking(digits,numString,0);
        return list;
    }
    StringBuilder temp = new StringBuilder();
    public void backtracking(String dights,String[] numString,int num ){
        if(num == dights.length()){
            list.add(temp.toString());
            return;
        }
        String str = numString[dights.charAt(num) - '0'];
        for (int i =0 ; i < str.length(); i++){
            temp.append(str.charAt(i));
            backtracking(dights,numString,num+1);
            temp.deleteCharAt(temp.length() - 1);
        }
    }
}