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Arpit Rathore
Arpit Rathore

Posted on • Edited on

Java Cheat sheet for leetcode

Data Structures Basics

1. Arrays 

int[] arr = new int[10];
arr[1] = 34;
arr[1] = 35;

int i = arr[1];
int size = arr.length;

// Fill array with a value
Arrays.fill(arr, -1);

// Sort
Arrays.sort(arr); // Ascending
Arrays.sort(arr, (a, b) -> b - a); // Descending
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2. Character 

char c1 = 'a';
char[] arr = "test".toCharArray();
char c2 = str.charAt(2);

// Character
Character.toLowerCase(c)
Character.isLowerCase(c);

Character.toUpperCase(c)
Character.isUpperCase(c);

Character.isLetter(c)
Character.isDigit(c)
Character.isAlphabetic(c);
Character.isLetterOrDigit(c)
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3. String and StringBuilder 

// String
String str = new String();
str = "abc";
int size = str.length();
char[] arr = str.toCharArray();
String sub = str.substring(1, 3); //[a, b)
str.toLowerCase();
str.toUpperCase();
str.replaceAll("a", "z");

// String Builder
StringBuilder sb = new StringBuilder();
sb.append("hellohowareyou?");
sb.insert(2, "z");
sb.delete(1, 3) // [from, to)
sb.deleteCharAt(0);
sb.reverse();
sb.substring(1);
sb.substring(3, 5); // [from, to)
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4. List and ArrayList 

List<Integer> list = new ArrayList<>();
list.add(5);
list.addAll(List.of(6, 7, 8, 9, 10));

// Index based
list.remove(3);
list.set(0, 9);
list.get(0);

int size = list.size();
list.clear();
list.isEmpty();

// Inefficient - O(n)
list.contains(77);
list.indexOf(10);

Integer[] arr = list.toArray(new Integer[0]);
int[] arrPrimitive = list.stream().mapToInt(Integer::intValue).toArray();

// Sort O(nlog(n))
Collections.sort(list);
Collections.sort(list, (a, b) -> b - a);
Collections.sort(personList, (p1, p2) -> p2.age - p1.age);
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5. Set 

Stack<Integer> st = new Stack<>();
st.push(1);
st.pop();

st.peek();
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6. Queue and Deque 

// -----------------Queue-----------------
Queue<Integer> q = new LinkedList<>();
q.add(1);
q.remove();
q.peek();


// Without exception
q.offer(2);
q.poll()

// -----------------Deque-----------------

Deque<Integer> dq = new LinkedList<>();
dq.addFirst(1);
dq.addLast(2);

int f1 = dq.getFirst();
int l1 = dq.getLast();

int f2 = dq.removeFirst();
int l2 = dq.removeLast();
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7. PriorityQueue 

// Basics
PriorityQueue<Integer> pq = new PriorityQueue<>();
PriorityQueue<Integer> maxPQ = new PriorityQueue<>(Comparator.reverseOrder());

// Add/Offer
pq.add(5);

// Remove/Poll
Integer item = pq.remove();

// Return without removing
Integer item = pq.peek();

// -----------------Inefficient-----------------
// Contains
boolean exists = pq.contains(5);

// Remove specific object
pq.remove(5);

//--------------Custom comparators--------------------

// Max heap
PriorityQueue<Integer> maxHeap = new PriorityQueue<>(Comparator.reverseOrder());

// Based on string length
PriorityQueue<String> strPQByLength = new PriorityQueue<>((s1, s2) -> s2.length() - s1.length());

// Lexicographical Order
PriorityQueue<String> strPQLexi = new PriorityQueue<>();

// Reverse Lexicographical Order
PriorityQueue<String> strPQLexiDesc = new PriorityQueue<>(Comparator.reverseOrder());

// ----------------Custom Objects------------------

class Person {
    String name;
    int age;
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

// Based on age in descending order
PriorityQueue<Person> personPQ = new PriorityQueue<>((p1, p2) -> p2.age - p1.age);
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Visualization helper methods

I have created few helper methods to visualize a data structure during an algorithm.

1. Two pointers

  • int[] : Integer Array 
private void tpi(int[] s, int l, int r, String... prefix) {
    if (prefix.length == 0) {
        prefix = new String[] { "" };
    }
    System.out.print(prefix[0]);
    for (int i = 0; i < s.length; i++) {
        if (i == l && i == r) {
            System.out.printf("[(%s)]", s[i]);
        } else if (i == l) {
            System.out.printf("(%s)", s[i]);
        } else if (i == r) {
            System.out.printf("[%s]", s[i]);
        } else {
            System.out.print(s[i]);
        }
        System.out.print(" ");
    }
    System.out.printf("| l=%s, r=%s %n", l, r);
}
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  • char[] : Character Array 
// Two pointer char array
private void tpc(char[] s, int l, int r, String... prefix) {
    if (prefix.length == 0) {
        prefix = new String[] { "" };
    }
    System.out.print(prefix[0]);
    for (int i = 0; i < s.length; i++) {
        if (i == l && i == r) {
            System.out.printf("[(%s)]", s[i]);
        } else if (i == l) {
            System.out.printf("(%s)", s[i]);
        } else if (i == r) {
            System.out.printf("[%s]", s[i]);
        } else {
            System.out.print(s[i]);
        }
        System.out.print(" ");
    }
    System.out.printf("| l=%s, r=%s %n", l, r);
}
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  • String : String 
// Two pointer string
private void tps(String s, int l, int r, String... prefix) {
    if (prefix.length == 0) {
        prefix = new String[] { "" };
    }
    System.out.print(prefix[0]);
    for (int i = 0; i < s.length(); i++) {
        if (i == l && i == r) {
            System.out.printf("[(%s)]", s.charAt(i));
        } else if (i == l) {
            System.out.printf("(%s)", s.charAt(i));
        } else if (i == r) {
            System.out.printf("[%s]", s.charAt(i));
        } else {
            System.out.print(s.charAt(i));
        }
        System.out.print(" ");
    }
    System.out.printf("| l=%s, r=%s %n", l, r);
}
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2. Sliding Window

  • int[] : Integer Array 
// Sliding window int array
private void swi(int[] nums, int l, int r, String... prefix) {
    if (prefix.length == 0) {
        prefix = new String[] { "" };
    }
    System.out.print(prefix[0]);
    for (int i = 0; i < nums.length; i++) {
        if (i == l) {
            System.out.print("[");
        }
        System.out.print(nums[i]);
        if (i == r) {
            System.out.print("]");
        }
        System.out.print(" ");
    }
    System.out.printf("| l=%s, r=%s %n", l, r);
}
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  • char[] : Character Array 
// Sliding window char array
private void swi(char[] chars, int l, int r, String... prefix) {
    if (prefix.length == 0) {
        prefix = new String[] { "" };
    }
    System.out.print(prefix[0]);
    for (int i = 0; i < chars.length; i++) {
        if (i == l) {
            System.out.print("[");
        }
        System.out.print(chars[i]);
        if (i == r) {
            System.out.print("]");
        }
        System.out.print(" ");
    }
    System.out.printf("| l=%s, r=%s %n", l, r);
}
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  • String : String 
// Sliding window string
private void sws(String s, int l, int r, String... prefix) {
    if (prefix.length == 0) {
        prefix = new String[] { "" };
    }
    System.out.print(prefix[0]);
    for (int i = 0; i < s.length(); i++) {
        if (i == l) {
            System.out.print("[");
        }
        System.out.print(s.charAt(i));
        if (i == r) {
            System.out.print("]");
        }
        System.out.print(" ");
    }
    System.out.printf("| l=%s, r=%s %n", l, r);
}
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3. Binary Search 

// Print binary search
private void pbs(int[] a, int m, int l, int r, String... prefix) {
    if (prefix.length == 0) {
        prefix = new String[] { "" };
    }
    System.out.print(prefix[0]);
    System.out.printf("L: %s, M: %s, R: %s | ", l, m, r);
    for (int i = 0; i < a.length; i++) {
        if (i == m) {
            if (m == l && m == r) {
                System.out.printf("[({%s})]", a[i]);
            } else if (m == l) {
                System.out.printf("({%s})", a[i]);
            } else if (m == r) {
                System.out.printf("[{%s}]", a[i]);
            } else {
                System.out.printf("{%s}", a[i]);
            }
        } else if (i == l && i == r) {
            System.out.printf("[(%s)]", a[i]);
        } else if (i == l) {
            System.out.printf("(%s)", a[i]);
        } else if (i == r) {
            System.out.printf("[%s]", a[i]);
        } else {
            System.out.print(a[i]);
        }
        System.out.print(" ");
    }
    System.out.println();
}
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4. Two dimension Array

  • int[][] : Integer Array 
// Two dimension integer
private void tdi(int[][] a) {
    for (int[] row : a) {
        System.out.println(Arrays.toString(row));
    }
    System.out.println("---");
}
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  • char[][] : Character Array 
// Two dimension char
private void tdc(char[][] a) {
    for (char[] row : a) {
        System.out.println(Arrays.toString(row));
    }
    System.out.println("---");
}
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  • String[][] : String 
// Two dimension String
private void tds(String[][] a) {
    for (String[] row : a) {
        System.out.println(Arrays.toString(row));
    }
    System.out.println("---");
}
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5. Print Linked list 

// Print linked list
private void pll(ListNode head, String... prefix) {
    if (prefix.length == 0) {
        prefix = new String[] { "" };
    }
    System.out.print(prefix[0]);

    int size = 0;
    ListNode curr = head;
    while (curr != null) {
        size++;
        curr = curr.next;
    }

    curr = head;
    int i = 0;
    while (curr != null) {
        if (i == size / 2) {
            System.out.printf("[%s] -> ", curr.val);
        } else {
            System.out.printf("%s -> ", curr.val);
        }
        curr = curr.next;
        i++;
    }
    System.out.print("null\n");
}
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6. Print Binary Tree 

// Print binary tree
private void pbt(final TreeNode node) {
    String result = ppt(node, new StringBuilder(), true, new StringBuilder());
    System.out.println(result);
}

private String pbtUtil(TreeNode node, StringBuilder prefix, boolean isTail, StringBuilder sb) {
    if (node.right != null) {
      pbtUtil(node.right, new StringBuilder().append(prefix).append(isTail ? "│   " : "    "), false,
          sb);
    }
    sb.append(prefix).append(isTail ? "└── " : "┌── ").append(node.val).append("\n");
    if (node.left != null) {
      pbtUtil(node.left, new StringBuilder().append(prefix).append(isTail ? "    " : "│   "), true, sb);
    }
    return sb.toString();
}
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