[Leetcode] 251. Flatten 2D Vector 解題報告

題目：

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 returns false, the order of elements returned by next should be: 

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

.

Follow up:

As an added challenge, try to code it using only iterators in C++ or iterators in Java.

思路：

需要維護兩個動态疊代器row_it和col_it，分别表示目前位置的行和列。但是由于我們不存儲vec2d本身，是以我們還需要另外一個靜态疊代器row_end，表示整個二維數組的末尾。這裡需要特别注意的是在判斷hasNext的時候，要處理那種跳過空行的情況，請見下面的代碼實作。

代碼：

class Vector2D {
public:
    Vector2D(vector<vector<int>>& vec2d) {
        if (vec2d.size() == 0) {
            return;
        }
        row_it = vec2d.begin(), row_end = vec2d.end();
        col_it = (*row_it).begin();
    }

    int next() {
        return *col_it++;
    }

    bool hasNext() {
        while (row_it != row_end && col_it == (*row_it).end()) {
            ++row_it;
            col_it = (*row_it).begin();
        }
        return row_it != row_end;
    }
private:
    vector<vector<int>>::iterator row_it, row_end;
    vector<int>::iterator col_it;
};

/**
 * Your Vector2D object will be instantiated and called as such:
 * Vector2D i(vec2d);
 * while (i.hasNext()) cout << i.next();
 */