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import java.util.*;

/**
 * 20. Valid Parentheses
 * Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
 * The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.
 */
class Solution {
    public boolean isValid(String s) {
        Map<Character, Character> brackets = new HashMap<>();

bnyu

import java.util.HashMap;
import java.util.Map;

/**
 * 19. Remove Nth Node From End of List
 * Given a linked list, remove the nth node from the end of list and return its head. Given n will always be valid.
 */
class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        Map<Integer, ListNode> map = new HashMap<>();

bnyu

import java.util.ArrayList;
import java.util.List;

/**
 * 17. Letter Combinations of a Phone Number
 * Given a digit string, return all possible letter combinations that the number could represent.
 * A mapping of digit to letters (just like on the telephone buttons) is given below.
 */
class Solution {

bnyu

import java.util.*;

/**
 * 15. 3Sum
 * Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.
 * Note: The solution set must not contain duplicate triplets.
 */
class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        Map<Integer, Integer> numNum = new HashMap<>();

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/**
 * 14. Longest Common Prefix
 * Write a function to find the longest common prefix string amongst an array of strings.
 */
class Solution {
    public String longestCommonPrefix(String[] strs) {
        String ans = "";
        if (strs.length >= 1) {
            String refer = strs[0];
            int common = 0;

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/**
 * 11. Container With Most Water
 * You may not slant the container and n is at least 2.
 */
class Solution {
    public int maxArea(int[] height) {
        int area = 0;
        int length = height.length;
        int lHighest = 0;
        boolean leftIsHigher = false;

bnyu

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * 10. Regular Expression Matching
 * Implement regular expression matching with support for '.' and '*'.
 */
class Solution {

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import java.util.HashMap;
import java.util.Map;

/**
 * 9. Palindrome Number
 * Determine whether an integer is a palindrome. Do this without extra space.
 */
class Solution {
    public boolean isPalindrome(int x) {
        if (x < 0) {

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/**
 * 7. Reverse Integer
 * Reverse digits of an integer.
 */
class Solution {
    public int reverse(int x) {
        // 0x80000000 = ~0x80000000 + 1
        if (x == Integer.MIN_VALUE) {
            return 0;
        }

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class Solution {
    public String convert(String s, int numRows) {
        String ans;
        if (!s.isEmpty() && numRows > 0) {
            if (numRows == 1) {
                return s;
            }
            int length = s.length();
            char[] chars = new char[length];
            int lIndex;