题目地址：https://leetcode.com/problems/flatten-nested-list-iterator/description/
题目描述
Given a nested list of integers, implement an iterator to flatten it.
Each element is either an integer, or a list -- whose elements may also be integers or other lists.
Example 1:
Given the list [[1,1],2,[1,1]],
By calling next repeatedly until hasNext returns false,
the order of elements returned by next should be: [1,1,2,1,1].
Example 2:
Given the list [1,[4,[6]]],
By calling next repeatedly until hasNext returns false,
the order of elements returned by next should be: [1,4,6].
题目大意
生成一个嵌套的数组迭代对象的迭代器。
解题方法
递归+队列
该做法是作弊方法，因为题目想要我们给出的迭代器应该是对原始对象的迭代，而不是对自己新建对象的迭代。
这个做法需要我们设计一个数据结构保存嵌套数组的每个元素，我们选择了队列。
重点是利用递归把整个嵌套的列表迭代器给压平。注意，题目已经给了我们它的数据结构，而不是普通的list。所以我们必须用他的函数。题目中虽然是多重嵌套，但是归根到底，对于每层的嵌套都是一个一维数组而已。因此，不要想复杂，直接循环该一维数组，如果是整数，添加到队列中，如果是嵌套的列表则继续解嵌套。
最后的结果是有按照从左到右有序的，这个可以放心。
Python代码如下：
# """
# This is the interface that allows for creating nested lists.
# You should not implement it, or speculate about its implementation
# """
#class NestedInteger(object):
# def isInteger(self):
# """
# @return True if this NestedInteger holds a single integer, rather than a nested list.
# :rtype bool
# """
#
# def getInteger(self):
# """
# @return the single integer that this NestedInteger holds, if it holds a single integer
# Return None if this NestedInteger holds a nested list
# :rtype int
# """
#
# def getList(self):
# """
# @return the nested list that this NestedInteger holds, if it holds a nested list
# Return None if this NestedInteger holds a single integer
# :rtype List[NestedInteger]
# """
class NestedIterator(object):
def __init__(self, nestedList):
"""
Initialize your data structure here.
:type nestedList: List[NestedInteger]
"""
self.queue = collections.deque()
def getAll(nests):
for nest in nests:
if nest.isInteger():
self.queue.append(nest.getInteger())
else:
getAll(nest.getList())
getAll(nestedList)
def next(self):
"""
:rtype: int
"""
return self.queue.popleft()
def hasNext(self):
"""
:rtype: bool
"""
return len(self.queue)
# Your NestedIterator object will be instantiated and called as such:
# i, v = NestedIterator(nestedList), []
# while i.hasNext(): v.append(i.next())
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栈
上面说了事先保存所有对象的方法是个作弊的方法，为了不事先保存所有的对象，而是每次在调用hasNext()或者next()时迭代器向后移动，我们可以使用栈。用到栈的想法出发点是递归本身是用栈实现的。
栈存储的是NestedInteger类型，这样把vector中的元素倒序放进来，在hasNext()的判断过程中，如果看到当前的元素是Integer数据那么直接弹出；如果看到的是List类型，那么应该对其进行for循环，把里面的Integer再倒序放进来。
C++代码如下：
/**
* // This is the interface that allows for creating nested lists.
* // You should not implement it, or speculate about its implementation
* class NestedInteger {
* public:
* // Return true if this NestedInteger holds a single integer, rather than a nested list.
* bool isInteger() const;
*
* // Return the single integer that this NestedInteger holds, if it holds a single integer
* // The result is undefined if this NestedInteger holds a nested list
* int getInteger() const;
*
* // Return the nested list that this NestedInteger holds, if it holds a nested list
* // The result is undefined if this NestedInteger holds a single integer
* const vector<NestedInteger> &getList() const;
* };
*/
class NestedIterator {
public:
NestedIterator(vector<NestedInteger> &nestedList) {
for (int i = nestedList.size() - 1; i >= 0; --i) {
st.push(nestedList[i]);
}
}
int next() {
NestedInteger cur = st.top(); st.pop();
return cur.getInteger();
}
bool hasNext() {
while (!st.empty()) {
NestedInteger cur = st.top();
if (cur.isInteger()) {
return true;
}
st.pop();
for (int i = cur.getList().size() - 1; i >= 0; --i) {
st.push(cur.getList()[i]);
}
}
return false;
}
private:
stack<NestedInteger> st;
};
/**
* Your NestedIterator object will be instantiated and called as such:
* NestedIterator i(nestedList);
* while (i.hasNext()) cout << i.next();
*/
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