93.复原IP地址
题目
文章讲解
视频讲解
思路：每轮开始的位置需要变化就需要设置start
class Solution {List<String> result = new ArrayList<>();public List<String> restoreIpAddresses(String s) {if (s.length() < 4 ||s.length() > 12)return result;backTrack(s, 0, 0);return result;}private void backTrack(String s, int startIndex, int pointNum) {if (pointNum == 3) {if (isValid(s, startIndex, s.length() - 1)) {result.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++;backTrack(s, i + 2, pointNum);pointNum--;s = s.substring(0, i + 1) + s.substring(i + 2);} elsebreak;}}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;}
}
78.子集
题目
文章讲解
视频讲解
思路：
class Solution {List<List<Integer>> result = new ArrayList<>();List<Integer> path = new LinkedList<>();public List<List<Integer>> subsets(int[] nums) {backTracing(nums, 0);return result;}private void backTracing(int[] nums, int startIndex) {result.add(new ArrayList(path));//第一遍遍历为空集if (startIndex >= nums.length) {return;}for (int i = startIndex; i < nums.length; i++) {path.add(nums[i]);backTracing(nums, i + 1);path.removeLast();}}
}
90.子集II
题目
文章讲解
视频讲解
思路：利用used数组进行去重
问题：nums[i] == nums[i - 1]去重为什么不用nums[i] == nums[i +1] 解答，编程中的 i 所指都是当前的，要与前一个比较，当前与之前属于已知，后面的是未知。
class Solution {List<List<Integer>> result = new ArrayList<>();LinkedList<Integer> path = new LinkedList<>();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];backTracing(nums, 0);return result;}private void backTracing(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;}used[i] = true;path.add(nums[i]);backTracing(nums, i + 1);path.removeLast();used[i] = false;}}
}
)
