day27

1、leetcode93 复原IP地址

class Solution {
    List<String> res = new ArrayList<>();

    public boolean isValid(String 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++) {
            if (s.charAt(i) > '9' || s.charAt(i) < '0') {
                return false;
            }
            num = num * 10 + (s.charAt(i) - '0'); 
            if( num > 255) {
                return false;
            }
        }
        return true;
    }

    public void backTracking(String s, int startIndex, int pointNum) {
        if(pointNum == 3) {
            if (isValid(s,startIndex,s.length()-1)) {
                res.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);    //在str的后⾯插⼊⼀个逗点
                pointNum++;
                backTracking(s, i + 2, pointNum);// 插⼊逗点之后下⼀个⼦串的起始位置为i+2
                pointNum--;// 回溯
                s = s.substring(0, i + 1) + s.substring(i + 2);// 回溯删掉逗点
            } else {
                break;
            }
        }
    }

    public List<String> restoreIpAddresses(String s) {
        if (s.length() > 12) return res; // 算是剪枝了
        backTracking(s, 0, 0);
        return res;
    }
}

2、leetcode78 子集

class Solution {
    List<Integer> path = new LinkedList<Integer>();
    List<List<Integer>> res = new ArrayList<>();

    public void backTracking(int[] nums, int startIndex) {
        res.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.remove(path.size()-1);
        }
    }

    public List<List<Integer>> subsets(int[] nums) {
        backTracking(nums, 0);
        return res;
    }
}

3、leetcode90 子集Ⅱ

class Solution {
    List<Integer> path = new LinkedList<Integer>();
    List<List<Integer>> res = new ArrayList<>();

    public void backTracking(int[] nums, int startIndex) {
        res.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.remove(path.size()-1);
        }
    }

    public List<List<Integer>> subsetsWithDup(int[] nums) {
        Arrays.sort(nums);
        backTracking(nums, 0);
        return res;
    }
}