随想录训练营Day25 | 回溯 216.组合总和III, 17.电话号码的字母组合

标签： LeetCode闯关记

216.组合总和III

class Solution {
    List<List<Integer>> res = new ArrayList<>();
    LinkedList<Integer> path = new LinkedList<>();
    int sum = 0;
    public List<List<Integer>> combinationSum3(int k, int n) {
        findCombination(k,n,1);
        return res;

    }
    public void findCombination( int k, int targetSum,int startIndex ){
        //剪枝
        if(sum > targetSum){
            return;
        }
        // 终止条件
        if(path.size() == k){
            if(sum == targetSum){//cfQ77,多了一个判断条件
                res.add(new ArrayList<>(path));
                return;
            }
        }
        for (int i = startIndex; i <= 9 - (k - path.size()) + 1; i++) {
            path.add(i);
            sum += i;
            findCombination(k,targetSum,i + 1);
            sum -= i;
            path.removeLast();
        }
    }


}

17. Letter Combinations of a Phone Number

class Solution {
    List<String> res = new ArrayList<>();
    StringBuilder temp = new StringBuilder();

    public List<String> letterCombinations(String digits) {
        if(digits == null || digits.length() == 0){
            return res;
        }
        String[] num_letter =  {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };
        findLetters(digits,num_letter,0);
        return res;
    }
    public void findLetters(String digits, String[] num_letter, int index){
        if(temp.length() == digits.length()){
            res.add(temp.toString());
            return;
        }
        String str = num_letter[digits.charAt(index) - '0'];
        for (int i = 0; i < str.length(); i++) {
            temp.append(str.charAt(i));
            findLetters(digits,num_letter, index + 1);
            temp.deleteCharAt(temp.length() - 1);
        }
    }

}