【回溯】回溯问题的两个模板

模板一：

采用深度优先搜索

class Solution {
    List<Integer> temp = new ArrayList<Integer>();
    List<List<Integer>> ans = new ArrayList<List<Integer>>();

    public List<List<Integer>> combinationSum3(int k, int n) {
        dfs(1, 9, k, n);
        return ans;
    }

    public void dfs(int cur, int n, int k, int sum) {
        if (temp.size() + (n - cur + 1) < k || temp.size() > k) {
            return;
        }
        if (temp.size() == k) {
            int tempSum = 0;
            for (int num : temp) {
                tempSum += num;
            }
            if (tempSum == sum) {
                ans.add(new ArrayList<Integer>(temp));
                return;
            }
        }
        temp.add(cur);
        dfs(cur + 1, n, k, sum);
        temp.remove(temp.size() - 1);
        dfs(cur + 1, n, k, sum);
    }
}

模板二：

采用广度优先搜索

class Solution {

    List<List<Integer>> res = new ArrayList<>();
    List<Integer> tmp = new ArrayList<>();
    int curSum = 0;
    public List<List<Integer>> combinationSum3(int k, int n) {
        dfs(1,n,k);
        return res;
    }

    public void dfs(int cur,int n,int k){
        if(tmp.size() == k && curSum == n){
            res.add(new ArrayList<>(tmp));
        }
        for(int i = cur;i<=9;i++){
            tmp.add(i);
            curSum +=i;
            dfs(i+1,n,k);
            curSum -=i;
            tmp.remove(tmp.size()-1);
        }
    }
}