BiruLyu

public class Solution {
    public boolean isValidSudoku(char[][] board) {
        
        for(int i = 0; i < 9; i++){
            HashSet<Character> row = new HashSet<Character>();
            HashSet<Character> col = new HashSet<Character>();
            HashSet<Character> cube = new HashSet<Character>();
            for(int j = 0; j < 9; j++){
                if(board[i][j] != '.' && !row.add(board[i][j])){
                    return false;

BiruLyu

public class Solution {
    public int lengthOfLongestSubstring(String s) {
        int start = 0;
        int end = 0;
        HashMap<Character, Integer> dict = new HashMap<Character, Integer>();
        int res = 0;
        while(end < s.length()){
        	char c = s.charAt(end);
        	if( dict.containsKey(c) && start <= dict.get(c)){
        		start = dict.get(c) + 1;

BiruLyu

class Solution {
    public String minWindow(String s, String t) {
        if (s == null || s.length() < 1 || t == null || t.length() < 1) return "";
        int end = 0, start = 0, count = t.length();
        int[] vector = new int[256];
        for (char c : t.toCharArray()) {
            vector[c]++;
        }
        char[] str = s.toCharArray();
        String res = "";

BiruLyu

public class Solution {
    public String addBinary(String a, String b) {
        StringBuilder sb = new StringBuilder();
        int i = a.length() - 1, j = b.length() -1, carry = 0;
        while (i >= 0 || j >= 0) {
            int sum = carry;
            if (j >= 0) sum += b.charAt(j--) - '0';
            if (i >= 0) sum += a.charAt(i--) - '0';
            sb.append(sum % 2);
            carry = sum / 2;

BiruLyu

public class Solution {
    public int lengthOfLastWord(String s) {
    
        int res = 0;
    
        s = s.trim();
        if(s == "") return res;
        for(int i = s.length() - 1; i >= 0; i--){
            if(s.charAt(i) == ' '){
                res = s.length() - i - 1;

BiruLyu

class Solution {
    public List<String> fullJustify(String[] words, int maxWidth) {
        List<String> res = new ArrayList<>();
        List<String> buf = new ArrayList<>();
        int len = 0;
        for (String word: words) {
            if (buf.size() + len + word.length() > maxWidth) {
                res.add(justify(buf, len, maxWidth));
                buf = new ArrayList<>();
                len = 0;

BiruLyu

public class Solution {
    public boolean isPowerOfFour(int num) {
        //return ((num - 1) & num) == 0 && ( num - 1 ) % 3 == 0;
        // x^n - 1的因式分解一定含有(x-1). 因为x = 1是 x^n - 1 = 0的一个根
        return ((num - 1) & num) == 0 && (num & 0x55555555) != 0;
        //The basic idea is from power of 2, We can use "n&(n-1) == 0" to determine if n is power of 2. 
        //0x55555555 is to get rid of those power of 2 but not power of 4
        //so that the single 1 bit always appears at the odd position 
    }
}

BiruLyu

public class Solution {
    public int maxProduct(String[] words) {
        int res = 0;
        if(words == null || words.length == 0) return res;
        int len = words.length;
        int[] vectors = new int[len];
        for(int i = 0; i < len; i++){
            String temp = words[i];
            for(int j = 0; j < temp.length(); j++){
                vectors[i] |= 1 << (temp.charAt(j) - 'a');

BiruLyu

/*the SUM of Arithmetic sequence */
public class Solution {
    public int missingNumber(int[] nums) {
        int len = nums.length;
        int sum = (0 + len)*(len + 1) / 2;
        for(int i = 0; i < nums.length; i++){
            sum -= nums[i];
        }
        return sum;
    }

BiruLyu

public class Solution {
    public String longestPalindrome(String s) {
        
        String res = "";
        int resLen = 0;
        
        int len = s.length();
        int[][] dp = new int[len][len];
        
        for(int i = len - 1; i >= 0; i--){