Comment on page

900.RLE-Iterator

900. RLE Iterator

题目地址

题目描述

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

代码

Approach #1 Store Exhausted Position and Quantity

Time: O(N+Q) && Space: O(N)
We can store an index i and quantity q which represents that q elements of A[i] (repeated A[i+1] times) are exhausted.
class RLEIterator {
int[] A;
int i, q;
public RLEIterator(int[] A) {
this.A = A;
i = q = 0;
}
​
public int next(int n) {
while (i < A.length) {
if (q + n > A[i]) {
n = n - (A[i] - q);
q = 0;
i += 2;
} else {
q += n;
return A[i+1];
}
}
return -1;
}
}
​
/**
* Your RLEIterator object will be instantiated and called as such:
* RLEIterator obj = new RLEIterator(A);
* int param_1 = obj.next(n);
*/