ElninoFong

public class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> result = new ArrayList<>();
        if (k > n) {
            return result;
        }
        List<Integer> res = new ArrayList<Integer>();
        combine(n, k, result, res, 1);
        return result;
    }

ElninoFong

public class Solution {
    public List<List<Integer>> subsetsWithDup(int[] num) {
        List<List<Integer>> result = new ArrayList<>();
        if (num == null || num.length == 0) {
            return result;
        }
        
        Arrays.sort(num);
        List<Integer> res = new ArrayList<>();
        subsets(num, result, res, 0);

ElninoFong

public class Solution {
    public List<List<Integer>> subsets(int[] S) {
        List<List<Integer>> result = new ArrayList<>();
        if (S == null || S.length == 0) {
            return result;
        }
        
        Arrays.sort(S);
        List<Integer> res = new ArrayList<>();
        subsets(S, result, res, 0);

ElninoFong

/**
 * Definition for an interval.
 * public class Interval {
 *     int start;
 *     int end;
 *     Interval() { start = 0; end = 0; }
 *     Interval(int s, int e) { start = s; end = e; }
 * }
 */
public class Solution {

ElninoFong

/**
 * Definition for an interval.
 * public class Interval {
 *     int start;
 *     int end;
 *     Interval() { start = 0; end = 0; }
 *     Interval(int s, int e) { start = s; end = e; }
 * }
 */
public class Solution {

ElninoFong

public class Solution {
    public List<Integer> grayCode(int n) {
        List<Integer> result = new ArrayList<Integer>();
        if (n == 0) {
            result.add(0);
            return result;
        }
        List<Integer> res = grayCode(n - 1);
        result.addAll(res);
        for (int i = res.size() - 1; i >= 0; i--) {

ElninoFong

public class Solution {
    public boolean isPalindrome(int x) {
        if (x < 0) {
            return false;
        }
        
        int digit = 1;
        while (x / digit >= 10) {   // err: x >= 10 * digit, as 10*digit may overflow
            digit *= 10;
        }

ElninoFong

public class Solution {
    public int reverse(int x) {
        int res = 0;
        while (x != 0) {
            // handle overflow/underflow
            if (Math.abs(res) > Integer.MAX_VALUE / 10) {
                return 0;
            }
            res = res * 10 + x % 10;
            x /= 10;

ElninoFong

public class Solution {
    public int reverse(int x) {
        long res = 0;
        int sign = x >= 0 ? 1 : -1;
        x = Math.abs(x);
        while (x > 0) {
            res = res * 10 + x % 10;
            x /= 10;
        }
        

ElninoFong

public class Solution {
    public int climbStairs(int n) {
        if (n <= 0) {
            return 0;
        }
        
        int prev = 1;
        int curr = 1;
        while (--n > 0) {
            int tmp = curr;