[LeetCode] 251. Flatten 2D Vector

Design and implement an iterator to flatten a 2d vector. It should support the following operations: next and hasNext.

Example:

Vector2D iterator = new Vector2D([[1,2],[3],[4]]);

iterator.next(); // return 1
iterator.next(); // return 2
iterator.next(); // return 3
iterator.hasNext(); // return true
iterator.hasNext(); // return true
iterator.next(); // return 4
iterator.hasNext(); // return false

Notes:

1. Please remember to RESET your class variables declared in Vector2D, as static/class variables are persisted across multiple test cases. Please see here for more details.
2. You may assume that next() call will always be valid, that is, there will be at least a next element in the 2d vector when next() is called.

展开二维向量。

这也是一道考察iterator的题目，只不过是把iterate的对象变成了一个二维数组。比较偷懒的办法就是先把所有元素放入一个队列，然后遍历队列即可。

Java实现

class Vector2D {
    private Queue<Integer> queue = new LinkedList<>();

    public Vector2D(int[][] v) {
        for (int i = 0; i < v.length; i++) {
            for (int j = 0; j < v[i].length; j++) {
                queue.offer(v[i][j]);
            }
        }
    }

    public int next() {
        return queue.poll();
    }

    public boolean hasNext() {
        return !queue.isEmpty();
    }
}

/**
 * Your Vector2D object will be instantiated and called as such:
 * Vector2D obj = new Vector2D(v);
 * int param_1 = obj.next();
 * boolean param_2 = obj.hasNext();
 */

不使用队列也能做，无非考察的是你对遍历二维矩阵的坐标的敏感程度。注意这道题给的二维向量，每一行的元素个数并不一样，所以不能用类似遍历二维矩阵那样的方式去遍历。

Java实现

class Vector2D {
    int[][] v;
    int i = 0;
    int j = 0;

    public Vector2D(int[][] v) {
        this.v = v;
    }

    public int next() {
        if (hasNext()) {
            return v[i][j++];
        } else {
            return -1;
        }
    }

    public boolean hasNext() {
        while (i < v.length && j == v[i].length) {
            i++;
            j = 0;
        }
        return i < v.length;
    }
}

/**
 * Your Vector2D object will be instantiated and called as such:
 * Vector2D obj = new Vector2D(v);
 * int param_1 = obj.next();
 * boolean param_2 = obj.hasNext();
 */

LeetCode 题目总结