day23：回溯part3，继续组合问题

39.组合总和

class Solution {
    List<List<Integer>> ans = new ArrayList<>();
    List<Integer> path = new LinkedList<>();
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        Arrays.sort(candidates);
        backtracking(candidates, target, 0, 0);
        return ans;
    }
    public void backtracking(int[] candidates, int target, int sum, int startIndex) {
        if (sum == target) {
            ans.add(new ArrayList<>(path));
            return;
        }
        for (int i = startIndex; i < candidates.length; i++) {
            if (sum > target) break;
            path.add(candidates[i]);
            sum += candidates[i];
            backtracking(candidates, target, sum, i);
            path.removeLast();
            sum -= candidates[i];
        }
    }
}

40.组合总和 II

class Solution {
    List<List<Integer>> ans = new ArrayList<>();
    List<Integer> path = new LinkedList<>();
    boolean[] used;
    public List<List<Integer>> combinationSum2(int[] candidates, int target) {
        used = new boolean[candidates.length];
        Arrays.fill(used, false);
        Arrays.sort(candidates);
        backtracking(candidates, target, 0, 0);
        return ans;
    }
    public void backtracking(int[] candidates, int target, int sum, int startIndex) {
        if (sum == target) {
            ans.add(new ArrayList<>(path));
            return;
        }
        for (int i = startIndex; i < candidates.length; i++) {
            if (sum > target) break;
            // 判断是树层上重复还是树枝上重复，树层上重复才需去重，回溯回来used数组前一位应为false
            if (i > 0 && candidates[i] == candidates[i - 1] && !used[i - 1]) continue;
            path.add(candidates[i]);
            sum += candidates[i];
            used[i] = true;
            backtracking(candidates, target, sum, i + 1);
            path.removeLast();
            sum -= candidates[i];
            used[i] = false;
        }
    }
}

131.分割回文串

class Solution {
    List<List<String>> ans = new ArrayList<>();
    List<String> path = new LinkedList<>();
    public List<List<String>> partition(String s) {
        backtracking(s, 0);
        return ans;
    }
    private void backtracking(String s, int startIndex) {
        if (startIndex >= s.length()) {
            ans.add(new ArrayList<>(path));
            return;
        }
        for (int i = startIndex; i < s.length(); i++) {
            if (isPalindrome(s, startIndex, i))
                path.add(s.substring(startIndex, i + 1));
            else continue;
            backtracking(s, i + 1);
            path.removeLast();
        }
    }
    private boolean isPalindrome(String s, int start, int end) {
        for (int i = start, j = end; i < j; i++, j--) {
            if (s.charAt(i) != s.charAt(j))
                return false;
        }
        return true;
    }
}