Leecode刷题(五)

19. 删除链表的倒数第 N 个结点

/**
 * 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 removeNthFromEnd(ListNode head, int n) {
        ListNode cur = head;
        ListNode pre = null;
        while(cur!=null){
            n--;
            if(n==-1){
                pre=head;
            }
            if(n<-1){
                pre=pre.next;
            }
            cur=cur.next;
        }
        if (n>0){
            return head;
        }
        if(pre==null){
            return head.next;
        }
        pre.next=pre.next.next;
        return head;
    }
}

20. 有效的括号

import java.util.*;
class Solution {
    public boolean isValid(String s) {
       //1.创建一个栈用来存储左括号
    Stack<String> chars = new Stack<>();
    //2.从左往右遍历字符串，拿到每一个字符
    for (int i = 0; i < s.length(); i++) {  
    String currChar = s.charAt(i) + "";
    //3.判断该字符是不是左括号，如果是，放入栈中存储
        if (currChar.equals("(")|| currChar.equals("{") || currChar.equals("[")) {
            chars.push(currChar);
        } else if (currChar.equals(")") || currChar.equals("}") || currChar.equals("]")) {
        //4.判断该字符是不是右括号，如果不是，继续下一次循 环 //5.如果该字符是右括号，则从栈中弹出一个元素t；
            if(chars.isEmpty()){
                return false;
            }
            String t = chars.pop();
            if((t.equals("(" ) && currChar.equals(")")) || (t.equals("[") && currChar.equals("]")) || (t.equals("{") && currChar.equals("}"))){

            }else {
                return false;
            }

        }
    }
//7.循环结束后，判断栈中还有没有剩余的左括号，如果有，则不匹配，如果没有，则匹配
        if (chars.size() == 0) {
            return true;
        } else {
            return false;
        }
    }
}

21. 合并两个有序链表

/**
 * 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 mergeTwoLists(ListNode list1, ListNode list2) {
        if (list1 == null ) return list2;
        if (list2 == null ) return list1;
        ListNode head = list1.val >= list2.val ? list2 : list1;
        ListNode cur1 = head.next;
        ListNode cur2 = head == list1 ? list2 : list1;
        ListNode pre = head;
        while (cur2 != null && cur1 != null) {
            if (cur1.val >= cur2.val) {
                pre.next = cur2;
                cur2 = cur2.next;
            } else {
                pre.next = cur1;
                cur1 = cur1.next;
            }
            pre = pre.next;
        }
        pre.next = cur1 != null ? cur1 : cur2;
        return head;
    }
}