raymondino

	public static boolean checkPan(Node a, Node b) {
		if(a == null && b == null) return true;
		else if (a == null || b == null) return false;
		Node ra = reverseR(a);
		while(ra != null && b != null) {
			if(ra.v != b.v)
				return false;
			else {
				ra = ra.next;
				b = b.next;

raymondino

	public static Node findCircleStart(Node n) {
		if(n == null) return n;
		HashSet hs = new HashSet<Integer>();
		while(n != null) {
			if(hs.add(n.hashCode())) {
				n = n.next;
			}
			else
				return n;
		}

raymondino

    // reverse add
    public static Node add(Node a, Node b) {
        if(a == null && b == null) return null;
        if(a == null) return b;
        if(b == null) return a;
        Node sumHead = new Node(-1);
        Node sumListPtr = sumHead;
        int carry = 0;
        while(a != null || b != null) {
            int sum = (a == null ? 0: a.v) + (b == null ? 0: b.v) + carry;

raymondino

    public static Node partition(Node n, int v) {
        if(n == null) return null;
        Node smallHead = new Node(-1);
        Node smallHeadMover = smallHead;
        Node bigHead = new Node(-1);
        Node bigHeaderMover = bigHead;
        while(n != null) {
            if(n.v < v) {
                if(smallHead.next == null) { 
                    smallHead.next = n;

raymondino

    public static Node findLastK(Node n, int k) {
        if(n == null) return null;
        int counter = 0;
        Node slow = n;
        Node fast = n;
        while(fast != null && counter <= k) {
            fast = fast.next;
            counter++;
        }
        if(counter < k) return null; // k is longer than the list size

raymondino

    public static void setZero(int[][] m) {
        if(m == null) return;
        int d1 = m.length;
        int d2 = m[0].length;
        
        // get all the zero element's locations
        ArrayList<Zero> z = new ArrayList<Zero>();
        for(int i = 0; i < d1; i++) {
            for(int j = 0; j < d2; j++) {

raymondino

    public static void rotate(int[][] m) {
        if(m == null || m.length <= 1)
            return;
        // rotate by 90: do transponse, then flip the columns.
        // transpose: 
        for(int i = 0; i < m.length; i++) {
            for(int j = i; j < m.length; j++) {
                int temp = m[i][j];
                m[i][j] = m[j][i];
                m[j][i] = temp;

raymondino

    public static String compress(String t) {
        int i = 0; // fast pointer
        int j = 0; // slow pointer
        StringBuilder sb = new StringBuilder();
        while(i < t.length()) {
            if(t.charAt(j) == t.charAt(i)) i++;
            else {
                sb.append(t.charAt(j)).append(i-j);
                j = i;
            }

raymondino

	// if x.length() < t.length(), pointers start from the string tail
	public static String backReplace(String x, String t) {
		// null check
		if(x == null || t == null) return x;
		// convert x into char arrays so as to replace in place
		char[] ax = new char[x.length()];
		for(int i = 0; i < x.length(); i++) {
			ax[i] = x.charAt(i);
		}
		int i = x.length() - 1; // front pointer

raymondino

	// assume all the letters are small
	public static boolean checkPermutation(String a, String b) {
		if(a == null && b == null) return true;
		else if(a == null || b == null) return false;
		else if(a.length() != b.length()) return false;
		else {
			int[] aa = new int[26];
			int[] ab = new int[26];
			for(int i = 0; i < a.length(); i++) {
				aa[a.charAt(i) - 'a'] ++;