Day24 93. 复原 IP 地址 | 78. 子集 | 90. 子集 II

93. 复原 IP 地址 - 力扣（LeetCode）

import java.util.ArrayList;
import java.util.List;

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

    public List<String> restoreIpAddresses(String s) {
        if (s.length() > 12)
            return result; // 剪枝
        backTrack(s, 0, 0, "");
        return result;
    }

    private void backTrack(String s, int startIndex, int pointNum, String currentIp) {
        if (pointNum == 3) { // 当点的数量达到3时，检查最后一段是否有效
            if (isValid(s, startIndex, s.length() - 1)) {
                result.add(currentIp + s.substring(startIndex));
            }
            return;
        }

        for (int i = startIndex; i < Math.min(s.length(), startIndex + 3); i++) { // 最多取3位数字
            if (isValid(s, startIndex, i)) {
                backTrack(s, i + 1, pointNum + 1, currentIp + s.substring(startIndex, i + 1) + ".");
            } else {
                break;
            }
        }
    }

    private 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++) {
            num = num * 10 + (s.charAt(i) - '0');
            if (num > 255) {
                return false;
            }
        }
        return true;
    }
}

78. 子集 - 力扣（LeetCode）

class Solution {
    public List<List<Integer>> subsets(int[] nums) {
        List<List<Integer>> result = new ArrayList<>();
        generateSubsets(nums, 0, new ArrayList<>(), result);
        return result;
    }

    private static void generateSubsets(int[] nums, int index, List<Integer> current, List<List<Integer>> result) {
        if (index == nums.length) {
            result.add(new ArrayList<>(current));
            return;
        }
        // 不选择当前索引的元素
        generateSubsets(nums, index + 1, current, result);
        // 选择当前索引的元素
        current.add(nums[index]);
        generateSubsets(nums, index + 1, current, result);
        current.remove(current.size() - 1); // 回溯，撤销选择
    }
}

90. 子集 II - 力扣（LeetCode）

public class Solution {
    List<List<Integer>> result = new ArrayList<>();// 存放符合条件结果的集合
    LinkedList<Integer> path = new LinkedList<>();// 用来存放符合条件结果
    boolean[] used;

    public List<List<Integer>> subsetsWithDup(int[] nums) {
        if (nums.length == 0) {
            result.add(path);
            return result;
        }
        Arrays.sort(nums);
        used = new boolean[nums.length];
        subsetsWithDupHelper(nums, 0);
        return result;
    }

    private void subsetsWithDupHelper(int[] nums, int startIndex) {
        result.add(new ArrayList<>(path));
        if (startIndex >= nums.length) {
            return;
        }
        for (int i = startIndex; i < nums.length; i++) {
            if (i > 0 && nums[i] == nums[i - 1] && !used[i - 1]) {
                continue;
            }
            path.add(nums[i]);
            used[i] = true;
            subsetsWithDupHelper(nums, i + 1);
            path.removeLast();
            used[i] = false;
        }
    }
}

没有理解，先跳过吧。