NC_52_BRACKETS_CHECK NC_53_REMOVE_KTH_FROMEND NC_54_THREE_NUM_ZERO

package org.example.interview.practice;

import java.util.ArrayDeque;
import java.util.Deque;

/**
 * @author xianzhe.ma
 * @date 2021/7/24
 */

public class NC_52_BRACKETS_CHECK {

    public static boolean isValid (String s) {
        // write code here
        int length = s.length();
        Deque<Character> deque = new ArrayDeque<>();
        for (int i=0;i<length;i++) {
            char c = s.charAt(i);
            if (allowInsert(c)) {
                deque.push(c);
            } else {
                boolean flag = true;
                switch (c) {
                    case ')' :
                        if (!deque.isEmpty() && deque.peek() == '(') {
                            deque.pop();
                        } else {
                            flag = false;
                        };
                        break;
                    case ']' :
                        if (!deque.isEmpty() && deque.peek() == '[') {
                            deque.pop();
                        } else {
                            flag = false;
                        };
                        break;
                    case '}' :
                        if (!deque.isEmpty() && deque.peek() == '{') {
                            deque.pop();
                        } else {
                            flag = false;
                        };
                        break;
                    default:
                        flag = false;;
                        break;
                }
                if (!flag) {
                    return false;
                }
            }


        }
        return deque.isEmpty();

    }

    private static boolean allowInsert(char c) {
        if (c == '(' || c == '[' || c == '{') {
            return true;
        }
        return false;
    }

    public static void main (String[] args) {
        String str = "]";
        isValid(str);
    }
}

package org.example.interview.practice;

/**
 * @author xianzhe.ma
 * @date 2021/9/2
 */

public class NC_53_REMOVE_KTH_FROMEND {

    public static ListNode removeNthFromEnd (ListNode head, int n) {
        // write code here

        int length = 0;
        ListNode countNode = head;
        ListNode q = head;
        while (countNode != null) {
            length++;
            countNode = countNode.next;
        }

        if(length < 2){
            return null;
        }
        // 特殊情况
        if(n == length){
            return q.next;
        }

        ListNode cur = head;

//         int i=0;
        for (int i=0;i<n&& cur!=null;i++) {
            cur = cur.next;
        }

        ListNode pre = new ListNode(-1);
        pre.next = head;
        while (pre != null && cur != null) {
            pre = pre.next;
            cur = cur.next;
        }
        ListNode temp = pre.next.next;
        pre.next = temp;
        return head;

    }

    public static class ListNode {
        int val;
        ListNode next = null;

        ListNode(int val) {
            this.val = val;
        }
    }

    public static void main (String[] args) {
        ListNode node1 = new ListNode(1);

        removeNthFromEnd(node1, 1);

    }
}

package org.example.interview.practice;

import java.util.ArrayList;
import java.util.Arrays;

/**
 * @author xianzhe.ma
 * @date 2021/9/7
 */

public class NC_54_THREE_NUM_ZERO {

    public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();

        if (num == null || num.length < 3) {
            return res;
        }
        Arrays.sort(num);// 排序
        for (int i = 0; i < num.length - 2; i++) {
            if (num[i] > 0) {
                break;// 如果当前数字大于0，则三数之和一定大于0，所以结束循环
            }
            if (i > 0 && num[i] == num[i - 1]) {
                continue;// 去重
            }
            int L = i + 1;
            int R = num.length - 1;

            while (L < R) {
                int sum = num[i] + num[L] + num[R];
                if (sum == 0) {
                    ArrayList<Integer> list = new ArrayList<>();
                    list.add(num[i]);
                    list.add(num[L]);
                    list.add(num[R]);
                    res.add(list);

                    while (L < R && num[L] == num[L + 1]) {
                        L++;
                    }
                    while (L < R && num[R] == num[R - 1]) {
                        R--;
                    }
                    L++;
                    R--;
                } else if (sum > 0) {
                    R--;
                } else if (sum < 0) {
                    L++;
                }
            }
        }
        return res;
    }
}