1670. Design Front Middle Back Queue

Posted on Edited on In

1670. Design Front Middle Back Queue

Description

Difficulty: Medium

Related Topics: Array, Linked List, Design, Queue, Data Stream

Design a queue that supports push and pop operations in the front, middle, and back.

Implement the FrontMiddleBack class:

  • FrontMiddleBack() Initializes the queue.
  • void pushFront(int val) Adds val to the front of the queue.
  • void pushMiddle(int val) Adds val to the middle of the queue.
  • void pushBack(int val) Adds val to the back of the queue.
  • int popFront() Removes the front element of the queue and returns it. If the queue is empty, return -1.
  • int popMiddle() Removes the middle element of the queue and returns it. If the queue is empty, return -1.
  • int popBack() Removes the back element of the queue and returns it. If the queue is empty, return -1.

Notice that when there are two middle position choices, the operation is performed on the frontmost middle position choice. For example:

  • Pushing 6 into the middle of [1, 2, 3, 4, 5] results in [1, 2, <u>6</u>, 3, 4, 5].
  • Popping the middle from [1, 2, <u>3</u>, 4, 5, 6] returns 3 and results in [1, 2, 4, 5, 6].

Example 1:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Input:
["FrontMiddleBackQueue", "pushFront", "pushBack", "pushMiddle", "pushMiddle", "popFront", "popMiddle", "popMiddle", "popBack", "popFront"]
[[], [1], [2], [3], [4], [], [], [], [], []]
Output:
[null, null, null, null, null, 1, 3, 4, 2, -1]

Explanation:
FrontMiddleBackQueue q = new FrontMiddleBackQueue();
q.pushFront(1);   // [1]
q.pushBack(2);    // [1, 2]
q.pushMiddle(3);  // [1, 3, 2]
q.pushMiddle(4);  // [1, 4, 3, 2]
q.popFront();     // return 1 -> [4, 3, 2]
q.popMiddle();    // return 3 -> [4, 2]
q.popMiddle();    // return 4 -> [2]
q.popBack();      // return 2 -> []
q.popFront();     // return -1 -> [] (The queue is empty)

Constraints:

  • 1 <= val <= 109
  • At most 1000 calls will be made to pushFront, pushMiddle, pushBack, popFront, popMiddle, and popBack.

Hints/Notes

  • use 2 queues

Solution

Language: C++

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
class FrontMiddleBackQueue {
public:
    deque<int> left;
    deque<int> right;

    FrontMiddleBackQueue() {

    }

    void pushFront(int val) {
        left.push_front(val);
        if (left.size() > right.size()) {
            right.push_front(left.back());
            left.pop_back();
        }
    }

    void pushMiddle(int val) {
        if (left.size() == right.size()) {
            right.push_front(val);
        } else {
            left.push_back(val);
        }
    }

    void pushBack(int val) {
        right.push_back(val);
        if (left.size() + 2 == right.size()) {
            left.push_back(right.front());
            right.pop_front();
        }
    }

    int popFront() {
        if (right.empty()) {
            return -1;
        }
        if (left.empty()) {
            int result = right.front();
            right.pop_front();
            return result;
        }
        int result = left.front();
        left.pop_front();
        if (left.size() + 2 == right.size()) {
            int tmp = right.front();
            right.pop_front();
            left.push_back(tmp);
        }
        return result;
    }

    int popMiddle() {
        if (right.empty()) {
            return -1;
        }
        if (right.size() > left.size()) {
            int result = right.front();
            right.pop_front();
            return result;
        }
        int result = left.back();
        left.pop_back();
        return result;
    }

    int popBack() {
        if (right.empty()) {
            return -1;
        }
        int result = right.back();
        right.pop_back();
        if (right.size() < left.size()) {
            right.push_front(left.back());
            left.pop_back();
        }
        return result;
    }
};

/**
 * Your FrontMiddleBackQueue object will be instantiated and called as such:
 * FrontMiddleBackQueue* obj = new FrontMiddleBackQueue();
 * obj->pushFront(val);
 * obj->pushMiddle(val);
 * obj->pushBack(val);
 * int param_4 = obj->popFront();
 * int param_5 = obj->popMiddle();
 * int param_6 = obj->popBack();
 */