93.复原IP地址

题目链接

https://leetcode.cn/problems/restore-ip-addresses/description/

题目描述

思路

class Solution {
    List<String> list = new ArrayList<>();
    public List<String> restoreIpAddresses(String s) {
        if(s.length()>12) return list;
        backTraing(s,0,0);
        return list;
    }

    public void backTraing(String s,int startIndex,int pointnum){
        //终止条件
        if(pointnum>=3){
            if(isValid(s,startIndex,s.length()-1)){
                list.add(s);
            }
            return;
        }
        for (int i = startIndex; i < s.length(); i++) {
            if(isValid(s,startIndex,i)){
                s = s.substring(0,i+1) + '.' + s.substring(i+1);
                pointnum++;
                backTraing(s,i+2,pointnum);
                pointnum--;
                //传入 i + 2 ，是因为加入了一个 . 所以 i 的后一位需要移动两个
                s = s.substring(0,i+1) + s.substring(i+2);
            }else {
                break;
            }
        }
    }

    public Boolean isValid(String s,int startIndex,int end){
        if(startIndex>end)return false;
        if(s.charAt(startIndex)=='0'&&startIndex!=end) return false;
        int num = 0;
        for (int i = startIndex; i <= end; i++) {
            if(s.charAt(i)<'0'||s.charAt(i)>'9') return false;
            num=num*10+(s.charAt(i)-'0');
            if(num>255){
                return false;
            }
        }
        return true;
    }
}

78.子集

题目链接

https://leetcode.cn/problems/subsets/

题目描述

思路

class Solution {
    List<List<Integer>> list = new ArrayList<>();
    List<Integer> list1 = new ArrayList<>();
    public List<List<Integer>> subsets(int[] nums) {
        if(nums.length==0||nums==null){
            list.add(new ArrayList<>(list1));
            return list;
        }
        backTracing(nums,0);
        return list;
    }

    public void backTracing(int[] nums,int startIndex){
        list.add(new ArrayList<>(list1));
        if(startIndex>=nums.length) return;
        for (int i = startIndex; i < nums.length; i++) {
            list1.add(nums[i]);
            backTracing(nums,i+1);
            list1.remove(list1.size()-1);
        }
    }
}

90.子集II

题目链接

https://leetcode.cn/problems/subsets-ii/description/

题目描述

思路

只是凭借前边几题的感觉写出来的，感觉还是没有真正会

class Solution {
    List<List<Integer>> list = new ArrayList<>();
    List<Integer> list1 = new ArrayList<>();
    boolean[] arr;
    public List<List<Integer>> subsetsWithDup(int[] nums) {
        arr = new boolean[nums.length];
        if(nums.length==0){
            list.add(new ArrayList<>(list1));
            return list;
        }
        Arrays.sort(nums);
        Arrays.fill(arr,false);
        backTracking(nums,0);
        return list;
    }

    public void backTracking(int[] nums,int startIndex){
        list.add(new ArrayList<>(list1));
        if(startIndex>=nums.length) return;
        for (int i = startIndex; i < nums.length; i++) {
            if(i>0&&nums[i] == nums[i-1]&&!arr[i-1]) continue;
            arr[i] = true;
            list1.add(nums[i]);
            backTracking(nums,i+1);
            list1.remove(list1.size()-1);
            arr[i] = false;
        }
    }
}