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力扣Hot100每日N题（19~24）

news 2025/6/18 11:06:03

42. 环形链表 II

找规律

public class Solution {public ListNode detectCycle(ListNode head) {ListNode fast = head, low = head;while(fast != null){low = low.next;if(fast.next == null) return null;fast = fast.next.next;if(fast == low){ListNode temp = head;while(temp != low){temp = temp.next;low = low.next;}return temp;}}return null;}
}

141. 环形链表

public class Solution {public boolean hasCycle(ListNode head) {ListNode fast = head, low = head;if(fast == null || fast.next == null) return false;while(fast != null){low = low.next;fast = fast.next;if(fast != null) fast = fast.next;if(fast == low) return true;}return false;}
}

139. 单词拆分

class Solution {public boolean wordBreak(String s, List<String> wordDict) {Set<String> wordDictSet = new HashSet<>(wordDict);boolean[] dp = new boolean[s.length() + 1];dp[0] = true;for(int i = 1; i <= s.length(); i++){for(int j = 0; j < i; j++){if(dp[j] && wordDictSet.contains(s.substring(j, i))){dp[i] = true;break;}}}return dp[s.length()];}
}

136. 只出现一次的数字

class Solution {public int singleNumber(int[] nums) {int result = 0;for (int i = 0; i < nums.length; i++) {result ^= nums[i];}return result;}
}

647. 回文子串

class Solution {public int countSubstrings(String s) {int ans = 0;for(int i = 0 ; i < s.length(); i++){ans += get(s, i, i) + get(s, i, i + 1);}return ans;}public int get(String s, int l, int r){int ans = 0;while(l >= 0 && r < s.length() && s.charAt(l) == s.charAt(r)){ans++;l--;r++;}return ans;}
}

128. 最长连续序列

class Solution {public int longestConsecutive(int[] nums) {Set<Integer> map = new HashSet<>();for(int num : nums) map.add(num);int ans = 0;for(int num : map){if(map.contains(num - 1)) continue;int len = 1, now = num;while(map.contains(now + 1)){len++;now++;}ans = Math.max(ans, len);}return ans;}
}