滑动窗口438.找到字符串中所有的字母异位词|链表|206.反转链表92.反转链表II

438.找到字符串中所有的字母异位词

思路：固定窗口

点击查看代码

class Solution {
    public List<Integer> findAnagrams(String s, String p) {
        List<Integer> result = new ArrayList<>();
        if (s.length() < p.length()) {
            return result;
        }
        // 统计p
        Map<Character, Integer> needMatch = new HashMap<>();
        // 统计window
        Map<Character, Integer> window = new HashMap<>();
        
        // 初始化needMatch
        for (char c : p.toCharArray()) {
            needMatch.put(c, needMatch.getOrDefault(c, 0) + 1);
        }
        
        int left = 0;
        int match = 0;
    
        for(int right = 0; right < s.length(); right++){
            char c = s.charAt(right);
            char l = s.charAt(left);
            //2hash
            if(needMatch.containsKey(c)){
                window.put(c, window.getOrDefault(c,0) + 1);
                if(needMatch.get(c).intValue() == window.get(c).intValue()){
                    match++;
                }
            }
            if(right-left + 1 == p.length()){
                if(match == needMatch.size()){
                    result.add(left);
                }
                //2hash
                if(needMatch.containsKey(l)){
                    if(needMatch.get(l).intValue() == window.get(l).intValue()){
                        match--;
                    }
                    window.put(l, window.get(l) - 1);
                }
                left++;
            }
        }
        return result;
    }
}

反转链表

思路：pre指针，cur指针，记录next。更改当前的，移动pre和cur。

点击查看代码

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode pre = null;
        ListNode cur = head;
        while(cur != null){
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }
}

92.反转链表II

点击查看代码

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseBetween(ListNode head, int left, int right) {
        ListNode dummy = new ListNode(0, head);
        ListNode p0 = dummy;

        for(int i = 0; i < left - 1; i++){
            p0 = p0.next;
        }

        ListNode pre = null;
        ListNode cur = p0.next;
        for(int i = 0; i < right - left + 1; i++){
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        p0.next.next = cur;
        p0.next = pre;

        return dummy.next;
    }
}