算法训练营Day28(回溯)

class Solution {
    List<String> result = new ArrayList<>();
    public List<String> restoreIpAddresses(String s) {
        backTracking(s,0,0);
        return result;
    }
    void backTracking(String s,int startIndex,int pointSum){
        if(pointSum==3){
            if(isvaild(s,startIndex,s.length()-1)){
                result.add(s);
            }
            return;
        }

        for(int i = startIndex;i<s.length();i++){
            if(isvaild(s,startIndex,i)){
                s = s.substring(0, i + 1) + "." + s.substring(i + 1);
                pointSum++;
                backTracking(s,i+2,pointSum);
                s = s.substring(0, i + 1) + s.substring(i + 2);
                pointSum--;
            }else{
                break;
            }
        }
    }

     // 判断字符串s在左闭?闭区间[start, end]所组成的数字是否合法
    private Boolean isvaild(String s, int start, int end) {
        if (start > end) {
            return false;
        }
        if (s.charAt(start) == '0' && start != end) { // 0开头的数字不合法
            return false;
        }
        int num = 0;
        for (int i = start; i <= end; i++) {
            if (s.charAt(i) > '9' || s.charAt(i) < '0') { // 遇到?数字字符不合法
                return false;
            }
            num = num * 10 + (s.charAt(i) - '0');
            if (num > 255) { // 如果?于255了不合法
                return false;
            }
        }
        return true;
    }
}

class Solution {
    List<List<Integer>> result = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    public List<List<Integer>> subsets(int[] nums) {
        backTracking(nums,0);
        return result;
    }
    void backTracking(int [] nums,int startIndex){
        result.add(new ArrayList(path));
        if(startIndex==nums.length){
            return;
        }
        for(int i = startIndex;i<nums.length;i++){
            path.add(nums[i]);
            backTracking(nums,i+1);
            path.removeLast();
        }
    }
}

class Solution {
     List<List<Integer>> result = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    public List<List<Integer>> subsetsWithDup(int[] nums) {
        Arrays.sort(nums);
        backTracking(nums,0);
        return result;
    }
    void backTracking(int [] nums,int startIndex){
        result.add(new ArrayList<>(path));
        // if(startIndex==nums.length){
        //     return;
        // }
        for(int i = startIndex;i<nums.length;i++){

            if(i>startIndex&&nums[i]==nums[i-1]){
                continue;
            }

            path.add(nums[i]);
            backTracking(nums,i+1);
            path.removeLast();
        }
    }
}