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前言

一、力扣93. 复原 IP 地址

class Solution {
    List<String> res = new ArrayList<>();
    StringBuilder sb;
    public List<String> restoreIpAddresses(String s) {
        StringBuilder sb = new StringBuilder(s);
        fun(sb, 0, 0);
        return res;
    }
    public void fun(StringBuilder s, int start, int depth){
        if(depth == 3){
            if(flag(s, start, s.length()-1)){
                res.add(s.toString());
            }
            return;
        }
        for(int i = start; i < s.length(); i ++){
            if(flag(s, start, i)){
                s.insert(i+1, '.');
                fun(s, i+2, depth+1);
                s.deleteCharAt(i+1);
            }else{
                break;
            }
        }
    }
    public boolean flag(StringBuilder s, int start, int end){
        if(start > end)
            return false;
        if(s.charAt(start) == '0' && start != end)
            return false;
        int num = 0;
        for(int i = start; i <= end; i++){
            int digit = s.charAt(i) - '0';
            num = num * 10 + digit;
            if(num > 255)
                return false;
        }
        return true;
    }
}

二、力扣78. 子集

class Solution {
    List<List<Integer>> res = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    public List<List<Integer>> subsets(int[] nums) {
        res.add(new ArrayList<>(path));
        fun(nums, 0);
        return res;
    }
    public void fun(int[] nums, int start){
        for(int i = start; i < nums.length; i ++){
            path.add(nums[i]);
            res.add(new ArrayList<>(path));
            fun(nums, i+1);
            path.remove(path.size()-1);
        }
    }
}

三、力扣90. 子集 II

class Solution {
    List<List<Integer>> res = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    public List<List<Integer>> subsetsWithDup(int[] nums){
        Arrays.sort(nums);
        fun(nums, 0);
        return res;
    }
    public void fun(int[] nums, int start){
        res.add(new ArrayList<>(path));
        if(start == nums.length){
            return ;
        }
        for(int i = start; i < nums.length; i ++){
            if(i > start && nums[i] == nums[i-1]){
                continue;
            }
            path.add(nums[i]);
            fun(nums, i+1);
            path.remove(path.size()-1);
        }
    }
}