算法修炼Day28|93.复原IP地址 ● 78.子集 ● 90.子集II

题目：93. 复原 IP 地址 - 力扣（LeetCode）

代码实现：

class Solution {
    List<String> ans = new ArrayList<>();
    public List<String> restoreIpAddresses(String s) {
        if (s.length() > 12) return ans;
        backtracking(s, 0, 0);
        return ans;
    }
    private void backtracking(String s, int startIndex, int pointNum) {
        if (pointNum == 3) {
            if (isValid(s, startIndex, s.length() - 1)) {
                ans.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++;
                backtracking(s, i + 2, pointNum); // 因为加了逗点，此时startIndex 为 i + 2 
                pointNum--;
                s = s.substring(0, i + 1) + s.substring(i + 2);
            } 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++) {
            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;
    }
}

题目：78. 子集 - 力扣（LeetCode）

代码实现：

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

题目：90. 子集 II - 力扣（LeetCode）

代码实现：

class Solution {
    List<List<Integer>> ans = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    public List<List<Integer>> subsetsWithDup(int[] nums) {
        Arrays.sort(nums);
        backtracking(nums, 0);
        return ans;
    }
    private void backtracking(int[] nums, int startIndex) {
        ans.add(new ArrayList<>(path));

        for (int i = startIndex; i < nums.length; i++) {
            // 去重逻辑
            if (i > startIndex && nums[i - 1] == nums[i]) continue;
            path.add(nums[i]);
            backtracking(nums, i + 1);
            path.remove(path.size() - 1);
        }
    }
}