代码随想录回溯算法总结

77.组合

class Solution {
    List<List<Integer>> res = new ArrayList();
    Deque<Integer> path = new ArrayDeque();
    public List<List<Integer>> combine(int n, int k) {
        combineHelper(n, k, 1);
        return res;
    }
    private void combineHelper(int n, int k, int startIndex) {
        if (path.size() == k) {
            res.add(new ArrayList<>(path));
            return;
        }
        for (int i = startIndex; i <= n - (k - path.size()) + 1; i++) {
            path.addFirst(i);
            combineHelper(n, k, i + 1);
            path.removeFirst();
        }
    }
}

216.组合总和III

class Solution {
    List<List<Integer>> res = new ArrayList();
    Deque<Integer> path = new ArrayDeque();
    int sum = 0;
    public List<List<Integer>> combinationSum3(int k, int n) {
        combinationSum3Helper(k, n, 1);
        return res;
    }
    private void combinationSum3Helper(int k, int n, int startIndex) {
        if (path.size() == k) {
            if (sum == n) {
                res.add(new ArrayList<>(path));
            }
            return;
        }
        for (int i = startIndex; i <= 9; i++) {
            if (sum + i > n) break;
            sum += i;
            path.addFirst(i);
            combinationSum3Helper(k, n, i + 1);
            path.removeFirst();
            sum -= i;
        }
    }
}

39. 组合总和

class Solution {
    List<List<Integer>> res = new ArrayList();
    Deque<Integer> path = new ArrayDeque();
    int sum = 0;
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        if (candidates == null || candidates.length == 0) return res;
        Arrays.sort(candidates);
        combinationSumHelper(candidates, target, 0);
        return res;
    }
    private void combinationSumHelper(int[] candidates, int target, int startIndex) {
        if (sum == target) {
            res.add(new ArrayList<>(path));
            return;
        }
        for (int i = startIndex; i < candidates.length; i++) {
            if (sum + candidates[i] > target) break;
            sum += candidates[i];
            path.addFirst(candidates[i]);
            combinationSumHelper(candidates, target, i);
            path.removeFirst();
            sum -= candidates[i];
        }
    }
}

40.组合总和II

class Solution {
    List<List<Integer>> res = new ArrayList();
    Deque<Integer> path = new ArrayDeque();
    int sum = 0;
    public List<List<Integer>> combinationSum2(int[] candidates, int target) {
        if (candidates == null || candidates.length == 0) return res;
        Arrays.sort(candidates);
        combinationSum2Helper(candidates, target, 0);
        return res;
    }
    private void combinationSum2Helper(int[] candidates, int target, int startIndex) {
        if (sum == target) {
            res.add(new ArrayList<>(path));
            return;
        }
        for (int i = startIndex; i < candidates.length; i++) {
            if (i > startIndex && candidates[i] == candidates[i - 1]) continue;
            if (sum + candidates[i] > target) break;
            sum += candidates[i];
            path.addFirst(candidates[i]);
            combinationSum2Helper(candidates, target, i + 1);
            path.removeFirst();
            sum -= candidates[i];
        }
    }
}

17.电话号码的字母组合

class Solution {
    List<String> res = new ArrayList();
    String[] numString = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    StringBuilder builder = new StringBuilder();
    public List<String> letterCombinations(String digits) {
        if (digits == null || digits.length() == 0) {
            return res;
        }
        letterCombinationsHelper(digits, 0);
        return res;
    }
    private void letterCombinationsHelper(String digits, int index) {
        if (index == digits.length()) {
            res.add(builder.toString());
            return;
        }
        String s = numString[digits.charAt(index) - '0'];
        for (int i = 0; i < s.length(); i++) {
            builder.append(s.charAt(i));
            letterCombinationsHelper(digits, index + 1);
            builder.deleteCharAt(builder.length() - 1);
        }
    }
}

78.子集

class Solution {
    List<List<Integer>> res = new ArrayList();
    Deque<Integer> path = new ArrayDeque();
    public List<List<Integer>> subsets(int[] nums) {
        subsetsHelper(nums, 0);
        return res;
    }
    private void subsetsHelper(int[] nums, int startIndex) {
        res.add(new ArrayList<>(path));
        for (int i = startIndex; i < nums.length; i++) {
            path.addFirst(nums[i]);
            subsetsHelper(nums, i + 1);
            path.removeFirst();
        }
    }
}