一、 93. 复原 IP 地址

题目连接：https://leetcode.cn/problems/restore-ip-addresses/

class Solution {
    private List<String> result;
    private List<String> paths;
    private boolean isVail(String s) {
        if (s.length() == 1) return true;
        if (s.length() > 3) return false;
        if (s.charAt(0) == '0') return false;
        if (Integer.parseInt(s) > 255) return false;
        return true;
    }
    private void backTracking(String s, int startIndex) {
        if (paths.size() > 4) return;
        if (startIndex >= s.length() && paths.size() == 4) {
            //收集结果
            StringBuilder str = new StringBuilder();
            for (int i = 0; i < paths.size(); i++) {
                str.append(paths.get(i));
                if (i != paths.size() - 1) {
                    str.append(".");
                }
            }
            result.add(str.toString());
            return;
        }
        for (int i = startIndex; i < s.length(); i++){
            String splitStr = s.substring(startIndex, i + 1);
            if (!isVail(splitStr)) break;
            paths.add(splitStr);
            backTracking(s, i + 1);
            paths.remove(paths.size() - 1);
        }
    }
    public List<String> restoreIpAddresses(String s) {
        result = new ArrayList<>();
        paths = new ArrayList<>();
        if (s.length() >  12) return result;
        backTracking(s, 0);
        return result;
    }
}

二、 78. 子集

题目连接：https://leetcode.cn/problems/subsets/

class Solution {
    private List<Integer> paths;
    private List<List<Integer>> result;

    private void backTracking(int[] nums, int indexStart){
        result.add(new ArrayList<>(paths));
        if (indexStart >= nums.length) return;
        for (int i = indexStart; i < nums.length; i++){
            paths.add(nums[i]);
            backTracking(nums, i + 1);
            paths.remove(paths.size() - 1);
        }
    }
    public List<List<Integer>> subsets(int[] nums) {
        paths = new ArrayList<>();
        result = new ArrayList<>();
        backTracking(nums, 0);
        return result;
    }
}

三、 90. 子集 II

题目连接：https://leetcode.cn/problems/subsets-ii/

class Solution {
    private List<Integer> paths;
    private List<List<Integer>> result;
    private void backTacking(int[] nums, int startIndex, boolean[] uses) {
        result.add(new ArrayList<>(paths));
        if (startIndex >= nums.length) return;
        for (int i = startIndex; i < nums.length; i++){
            if (i > 0 && nums[i] == nums[i - 1] && !uses[i - 1]) continue;
            paths.add(nums[i]);
            uses[i] = true;
            backTacking(nums, i + 1, uses);
            uses[i] = false;
            paths.remove(paths.size() - 1);
        }
    }
    public List<List<Integer>> subsetsWithDup(int[] nums) {
        paths = new ArrayList<>();
        result = new ArrayList<>();
        Arrays.sort(nums);
        boolean[] uses = new boolean[nums.length];
        Arrays.fill(uses, false);
        backTacking(nums, 0, uses);
        return result;
    }
}