Java算法 leetcode简单刷题记录10

庆祝一下：大概花费了9天，我把所有leetcode Java 的简单题都刷完了，接下来开始冲刺中等和复杂；

简单题里用到的比较多的是字符串的处理，转换，拆分，替换，PriorityQueue 依次输出最大最小值；走楼梯等动态规划；

charAt
startsWith
toLowerCase()
replaceAll
split
indexOf
进制转换： Integer.valueOf(“0xff”,16); 十六进制转十进制
Integer.valueOf(“0101”,2); 二进制转十进制
十进制转二进制 Integer.toBinaryString(10)
split 特殊字符拆分需要加 “\”+“.”
char转int int a = (int)(‘9’-‘0’); 或者 int a = (int)(str.charAt(0)-‘0’);

PriorityQueue每次弹出队列里剩下的值中的最大值，最小值；

class Solution {public long pickGifts(int[] gifts, int k) {// 保证每次队列弹出最大值PriorityQueue<Integer> queue = new PriorityQueue<Integer>((obj1, obj2) -> {return obj2 - obj1;});// 每次弹出最小值/** PriorityQueue<Integer> queue = new PriorityQueue<Integer>((obj1, obj2) -> {return obj1 - obj2;});**/for (int gift : gifts) {queue.add(gift);}int i = 0;while (i < k && queue.size() > 0) {int q = queue.poll();queue.add((int) Math.sqrt(q));i++;}long sum = 0;while (queue.size() > 0) {sum += queue.poll();}return sum;}
}

一周中的第几天： https://leetcode.cn/problems/day-of-the-week/
这是第2遍写这道题，稍微有些不一样，之前是告诉1970.1.1 是星期四，问 days后是哪个日期及星期几？
这道题是已知1970.1.1 是星期四，问某个日期假如2023.11.20是星期几?
俩道题知识点一致，需要计算1970.1.1到该日期经过了多少年，月，日，总天数，然后求星期。

class Solution {public String dayOfTheWeek(int day, int month, int year) {int[] months = new int[] { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };String[] weeks = new String[] { "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday" };int year0 = 1970;int week0 = 3;int month0 = 1;int sum = 0;while (year0 < year) {int days = 365;if (isRunYear(year0)) {days = 366;}System.out.println(year0 + " " + " " + days + " " + sum);sum += days;year0++;}for (int i = 1; i < month; i++) {int days = months[i];if (isRunYear(year0) && i == 2) {days = 29;}// System.out.println(year0 + " " + i + " " + days + " " + sum);sum += days;}// System.out.println(" days: " + day + " " + sum);sum += day - 1; // 当前天占一天，也不算进去；if (year > 1970) {sum -= 1; // 1970一整年中365天，起始日元旦算1天，所以要减掉；}// 假设法，假设距离只有3天，则为周四周五周六，否则mod 7求余；int n = sum > 3 ? (sum - 3) % 7 : sum + week0;// System.out.println(n);return weeks[n];}public boolean isRunYear(int year) {if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0) {return true;}return false;}
}

一年中的第几天： https://leetcode.cn/problems/day-of-the-year/
判断闰月即可：整除4不被100整除，或者整除400
赎金信： https://leetcode.cn/problems/ransom-note/
字符串处理，
charAt
replaceAll
按分隔符拆分字符串： https://leetcode.cn/problems/split-strings-by-separator/
特殊字符，直接split会不起作用，需要用 \+"."等

class Solution {public List<String> splitWordsBySeparator(List<String> words, char separator) {List<String> res = new ArrayList<>();for (String str : words) {res.addAll(Arrays.asList(str.split("\\" + String.valueOf(separator))).stream().filter(x -> {return x.length() != 0;}).collect(Collectors.toList()));}return res;}
}

最长交替子数组： https://leetcode.cn/problems/longest-alternating-subarray/
看着比较简单，还是需要把情况都考虑仔细，改改改才能通过所有的case…

class Solution {public int alternatingSubarray(int[] nums) {if (nums.length < 1) {return -1;}int res = -1;for (int i = 0; i < nums.length - 1; i++) {int cnt = -1;for (int j = i + 1; j < nums.length; j++) {if (cnt == -1 && nums[j - 1] + 1 == nums[j]) {cnt = 2;continue;}System.out.println(j + " " + nums[j - 1] + " " + nums[j]);if (nums[j - 1] + 1 == nums[j] || nums[j - 1] - 1 == nums[j]) {if (j >= 2) {if (nums[j] == nums[j - 2]) {cnt++;} else {break;}}} else {break;}}// System.out.println("----else " + j + " " + cnt);res = Math.max(res, cnt);cnt = -1;}return res;}
}