Description

Write an iterator that iterates through a run-length encoded sequence.

The iterator is initialized by RLEIterator(int[] A), where A is a run-length encoding of some sequence. More specifically, for all even i, A[i] tells us the number of times that the non-negative integer value A[i+1] is repeated in the sequence.

The iterator supports one function: next(int n), which exhausts the next n elements (n >= 1) and returns the last element exhausted in this way. If there is no element left to exhaust, next returns -1 instead.

For example, we start with A = [3,8,0,9,2,5], which is a run-length encoding of the sequence [8,8,8,5,5]. This is because the sequence can be read as “three eights, zero nines, two fives”.

Example 1:

Input: ["RLEIterator","next","next","next","next"], [[[3,8,0,9,2,5]],[2],[1],[1],[2]]
Output: [null,8,8,5,-1]
Explanation: 
RLEIterator is initialized with RLEIterator([3,8,0,9,2,5]).
This maps to the sequence [8,8,8,5,5].
RLEIterator.next is then called 4 times:

.next(2) exhausts 2 terms of the sequence, returning 8.  The remaining sequence is now [8, 5, 5].

.next(1) exhausts 1 term of the sequence, returning 8.  The remaining sequence is now [5, 5].

.next(1) exhausts 1 term of the sequence, returning 5.  The remaining sequence is now [5].

.next(2) exhausts 2 terms, returning -1.  This is because the first term exhausted was 5,
but the second term did not exist.  Since the last term exhausted does not exist, we return -1.

Note:

  1. 0 <= A.length <= 1000
  2. A.length is an even integer.
  3. 0 <= A[i] <= 10^9
  4. There are at most 1000 calls to RLEIterator.next(int n) per test case.
  5. Each call to RLEIterator.next(int n) will have 1 <= n <= 10^9.

Solutions

  这里主要搞清楚题目是啥意思,Run-length Encoding 要明白是怎么回事,然后结合题意要理解,比如这里的例子:

# 输入:其实是编码后的字串
[3,8,0,9,2,5]

# 输出:编码前的字串
[8,8,8,5,5] 

  编码后的字串表是的含义是 [(3,8),(0,9),(2,5)],每个括号表示的是 (数字出现次数,对应的数字),那么题意就比较清楚了。

1. Stack

# Time: O(n)
# Space: O(n)
class RLEIterator:

    def __init__(self, A: List[int]):
        self.stack = []
        n = len(A)
        for i in range(0, n, 2):
            if A[i] > 0:
                self.stack.insert(0, (A[i], A[i+1]))

    def next(self, n: int) -> int:
        while n > 0:
            if self.stack:
                cnt, num = self.stack.pop()
                if n > cnt:
                    n -= cnt
                else:
                    res = num
                    cnt -= n
                    self.stack.append((cnt, num))
                    return res
            else:
                return -1
        
# Your RLEIterator object will be instantiated and called as such:
# obj = RLEIterator(A)
# param_1 = obj.next(n)

# 9/9 cases passed (36 ms)
# Your runtime beats 70.12 % of python3 submissions
# Your memory usage beats 100 % of python3 submissions (13.2 MB)

References

  1. 900. RLE Iterator
  2. 900-RLE 迭代器