LeetCode 251. Flatten 2D Vector

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本文介绍了一种实现二维向量迭代器的方法，通过两种不同的方式实现了一个可以遍历不规则二维向量的迭代器。一种方法使用了额外的空间来存储一维化的向量元素，另一种则直接操作原始二维向量的迭代器。

I didn't buy the locked questions (This question is copied from other blog.)

Implement an iterator to flatten a 2d vector.

For example,
Given 2d vector =

[
  [1,2],
  [3],
  [4,5,6]
]

By calling next repeatedly until hasNext returnsfalse, the order of elements returned by next should be:[1, 2, 3, 4, 5, 6].

2 Method: Extra O(n) and two pointers.

#include <vector>
#include <iostream>
using namespace std;

class Vector2D {
private:
    vector< vector<int> >::iterator i, iEnd;
    int j = 0;
public:
    Vector2D(vector< vector<int> >& vec2d) {
        i = vec2d.begin();
        iEnd = vec2d.end();
    }

    int next() {
        hasNext();
        return (*i)[j++];
    }

    bool hasNext() {
        while(i != iEnd && j == (*i).size()) {
            i++;
            j = 0;
        }
        return i != iEnd;
    }
};

// O(n) extra space.

class Vector2DII {
private:
    vector<int> innerVec;
    int j = 0;
public:
    Vector2DII(vector< vector<int> >& vec2d) {
        for(int i = 0; i < vec2d.size(); ++i) {
            for(int j = 0; j < vec2d[i].size(); ++j) {
                innerVec.push_back(vec2d[i][j]);
            }
        }
    }

    int next() {
        hasNext();
        return innerVec[j++];
    }

    bool hasNext() {
        return j == innerVec.size();
    }
};

int main(void) {
    vector< vector<int> > test{{1, 2}, {1}};
    Vector2DII myVector(test);
    cout << myVector.next() << endl;
    cout << myVector.next() << endl;
    cout << myVector.next() << endl;
}