Day25|216.组合总和III | 17.电话号码的字母组合

216.组合总和III

class Solution {
    LinkedList path = new LinkedList<>();
    List> res = new ArrayList<>();
    
    public List> combinationSum3(int k, int n) { 
   travelsal(k,n,0,1);
   return res;
    }
​
    private void travelsal(int k,int n,int sum,int startIndex)//travelsal是遍历的意思
    {
        if(path.size()>k) return ;
        if(sum > n ) return ;
        if(path.size()==k&&sum==n){
            res.add(new ArrayList<>(path));
            return ;
        }
        for(int i = startIndex;i<=9;i++){
            //当然你这里也可以写i< 9-(k - path.size())+1
            path.add(i);
            sum += i;
            travelsal(k,n,sum,i+1);
            sum -= i;
            path.removeLast();
        }
    }
}

17.电话号码的字母组合

class Solution {
​
    private List res = new ArrayList<>();
    public List letterCombinations(String digits) {
    if(digits==null||digits.length()==0){
    return res;
    }
    String[] numString = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    backTracking(digits,0,numString);
    return res;
    }
   StringBuilder sb = new StringBuilder();
    private void backTracking(String digits,int num,String[] numString){
    if(num == digits.length()){
    res.add(sb.toString());
    return ;
    }
    String str = numString[digits.charAt(num)-'0'];
    for(int i = 0;i