Today I have done Implement Stack using Queues and leetcode's August LeetCoding Challenge with cpp.

Implement Stack using Queues

Description

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

• void push(int x) Pushes element x to the top of the stack.
• int pop() Removes the element on the top of the stack and returns it.
• int top() Returns the element on the top of the stack.
• boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

• You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.
• Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue's standard operations.

Example 1:

Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False

Constraints:

• 1 <= x <= 9
• At most 100 calls will be made to push, pop, top, and empty.
• All the calls to pop and top are valid.

Follow-up: Can you implement the stack using only one queue?

Solution

class MyStack {
  queue<int> container;
public:
  /** Initialize your data structure here. */
  MyStack() { }
  
  /** Push element x onto stack. */
  void push(int x) {
    int sz = container.size();
    container.push(x);
    while(sz--) {
      container.push(container.front());
      container.pop();
    }
  }
  
  /** Removes the element on top of the stack and returns that element. */
  int pop() {
    int val = container.front();
    container.pop();
    return val;
  }
  
  /** Get the top element. */
  int top() {
    return container.front();
  }
  
  /** Returns whether the stack is empty. */
  bool empty() {
    return container.empty();
  }
};

// Accepted
// 16/16 cases passed (0 ms)
// Your runtime beats 100 % of cpp submissions
// Your memory usage beats 98.89 % of cpp submissions (6.7 MB)

August LeetCoding Challenge 27

Description

Verify Preorder Serialization of a Binary Tree

One way to serialize a binary tree is to use preorder traversal. When we encounter a non-null node, we record the node's value. If it is a null node, we record using a sentinel value such as '#'.

For example, the above binary tree can be serialized to the string "9,3,4,#,#,1,#,#,2,#,6,#,#", where '#' represents a null node.

Given a string of comma-separated values preorder, return true if it is a correct preorder traversal serialization of a binary tree.

It is guaranteed that each comma-separated value in the string must be either an integer or a character '#' representing null pointer.

You may assume that the input format is always valid.

• For example, it could never contain two consecutive commas, such as "1,,3".

Note: You are not allowed to reconstruct the tree.

Example 1:

Input: preorder = "9,3,4,#,#,1,#,#,2,#,6,#,#"
Output: true

Example 2:

Input: preorder = "1,#"
Output: false

Example 3:

Input: preorder = "9,#,#,1"
Output: false

Constraints:

• 1 <= preorder.length <= 10^4
• preoder consist of integers in the range [0, 100] and '#' separated by commas ','.

Solution

auto speedup = [](){
  cin.tie(nullptr);
  cout.tie(nullptr);
  ios::sync_with_stdio(false);
  return 0;
}();
class Solution {
public:
  bool isValidSerialization(string preorder) {
    int count = 1;
    int len = preorder.length();
    for (int i = 0; i < len; ++i) {
      if(preorder[i] == '#') {
        count -= 1;
        i += 1;
        if(count == 0 && i != len) return false;
      } else {
        while(i < len && preorder[i] != ',') {
          i += 1;
        }
        count += 1;
      }
    }
    return !count;
  }
};

// Accepted
// 151/151 cases passed (0 ms)
// Your runtime beats 100 % of cpp submissions
// Your memory usage beats 66.91 % of cpp submissions (6.8 MB)