代码随想录一刷day25

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一、力扣216. 组合总和 III

class Solution {
    private List<List<Integer>> result = new ArrayList<>();
    private LinkedList<Integer> path = new LinkedList<>();
    private int sum = 0;
    public List<List<Integer>> combinationSum3(int k, int n) {
        fun(k , n, 1);
        return result;
    }
    public void fun(int k, int n, int startIndex){  
        if(path.size() == k && sum == n){
            result.add(new ArrayList(path));
            return ;
        }
        for(int i = startIndex; i <= 9; i ++){
            path.add(i);
            sum += i;
            fun(k, n, i+1);
            sum -= i;
            path.removeLast();
        }
    }
}

剪枝操作

class Solution {
    private List<List<Integer>> result = new ArrayList<>();
    private LinkedList<Integer> path = new LinkedList<>();
    private int sum = 0;
    public List<List<Integer>> combinationSum3(int k, int n) {
        fun(k , n, 1);
        return result;
    }
    public void fun(int k, int n, int startIndex){  
        if(sum > n)return ;
        if(path.size() == k && sum == n){
            result.add(new ArrayList(path));
            return ;
        }
        for(int i = startIndex; i <= 9; i ++){
            path.add(i);
            sum += i;
            fun(k, n, i+1);
            sum -= i;
            path.removeLast();
        }
    }
}

二、力扣17.电话号码的字母组合

class Solution {
    private List<String> list = new ArrayList<>();
    private StringBuilder temp = new StringBuilder();
    public List<String> letterCombinations(String digits) {
        if (digits == null || digits.length() == 0) {
            return list;
        }
        //初始对应所有的数字，为了直接对应2-9，新增了两个无效的字符串""
        String[] numString = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        fun(digits, numString, 0);
        return list;
    }
    public void fun(String digits,String[] numString , int num){
        if(num > digits.length() )return ;
        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));
            fun(digits, numString, num + 1);
            temp.deleteCharAt(temp.length()-1);
        }
    }
}

数据的函数和方法。