281. Zigzag Iterator

Description

Given two vectors of integers v1 and v2, implement an iterator to return their elements alternately.

Implement the ZigzagIterator class:

  • ZigzagIterator(List<int> v1, List<int> v2) initializes the object with the two vectors v1 and v2.
  • boolean hasNext() returns true if the iterator still has elements, and false otherwise.
  • int next() returns the current element of the iterator and moves the iterator to the next element.

 

Example 1:

Input: v1 = [1,2], v2 = [3,4,5,6]
Output: [1,3,2,4,5,6]
Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,3,2,4,5,6].

Example 2:

Input: v1 = [1], v2 = []
Output: [1]

Example 3:

Input: v1 = [], v2 = [1]
Output: [1]

 

Constraints:

  • 0 <= v1.length, v2.length <= 1000
  • 1 <= v1.length + v2.length <= 2000
  • -231 <= v1[i], v2[i] <= 231 - 1

 

Follow up: What if you are given k vectors? How well can your code be extended to such cases?

Clarification for the follow-up question:

The "Zigzag" order is not clearly defined and is ambiguous for k > 2 cases. If "Zigzag" does not look right to you, replace "Zigzag" with "Cyclic".

Follow-up Example:

Input: v1 = [1,2,3], v2 = [4,5,6,7], v3 = [8,9]
Output: [1,4,8,2,5,9,3,6,7]

Solutions

Solution 1

Python Code
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class ZigzagIterator:
    def __init__(self, v1: List[int], v2: List[int]):
        self.cur = 0
        self.size = 2
        self.indexes = [0] * self.size
        self.vectors = [v1, v2]

    def next(self) -> int:
        vector = self.vectors[self.cur]
        index = self.indexes[self.cur]
        res = vector[index]
        self.indexes[self.cur] = index + 1
        self.cur = (self.cur + 1) % self.size
        return res

    def hasNext(self) -> bool:
        start = self.cur
        while self.indexes[self.cur] == len(self.vectors[self.cur]):
            self.cur = (self.cur + 1) % self.size
            if self.cur == start:
                return False
        return True


# Your ZigzagIterator object will be instantiated and called as such:
# i, v = ZigzagIterator(v1, v2), []
# while i.hasNext(): v.append(i.next())

Java Code
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public class ZigzagIterator {
    private int cur;
    private int size;
    private List<Integer> indexes = new ArrayList<>();
    private List<List<Integer>> vectors = new ArrayList<>();

    public ZigzagIterator(List<Integer> v1, List<Integer> v2) {
        cur = 0;
        size = 2;
        indexes.add(0);
        indexes.add(0);
        vectors.add(v1);
        vectors.add(v2);
    }

    public int next() {
        List<Integer> vector = vectors.get(cur);
        int index = indexes.get(cur);
        int res = vector.get(index);
        indexes.set(cur, index + 1);
        cur = (cur + 1) % size;
        return res;
    }

    public boolean hasNext() {
        int start = cur;
        while (indexes.get(cur) == vectors.get(cur).size()) {
            cur = (cur + 1) % size;
            if (start == cur) {
                return false;
            }
        }
        return true;
    }
}

/**
 * Your ZigzagIterator object will be instantiated and called as such:
 * ZigzagIterator i = new ZigzagIterator(v1, v2);
 * while (i.hasNext()) v[f()] = i.next();
 */

Rust Code
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struct ZigzagIterator {
    v1: Vec<i32>,
    v2: Vec<i32>,
    /// `false` represents `v1`, `true` represents `v2`
    flag: bool,
}

impl ZigzagIterator {
    fn new(v1: Vec<i32>, v2: Vec<i32>) -> Self {
        Self {
            v1,
            v2,
            // Initially beginning with `v1`
            flag: false,
        }
    }

    fn next(&mut self) -> i32 {
        if !self.flag {
            // v1
            if self.v1.is_empty() && !self.v2.is_empty() {
                self.flag = true;
                let ret = self.v2.remove(0);
                return ret;
            }
            if self.v2.is_empty() {
                let ret = self.v1.remove(0);
                return ret;
            }
            let ret = self.v1.remove(0);
            self.flag = true;
            return ret;
        } else {
            // v2
            if self.v2.is_empty() && !self.v1.is_empty() {
                self.flag = false;
                let ret = self.v1.remove(0);
                return ret;
            }
            if self.v1.is_empty() {
                let ret = self.v2.remove(0);
                return ret;
            }
            let ret = self.v2.remove(0);
            self.flag = false;
            return ret;
        }
    }

    fn has_next(&self) -> bool {
        !self.v1.is_empty() || !self.v2.is_empty()
    }
}