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].

将1个含有整数元素的嵌套链表压平,就是把所以元素按嵌套关系变成1个list。按题目要求要有next和hasNext两个函数。

Java:

public class NestedIterator implements Iterator<Integer> {
Stack<NestedInteger> stack; public NestedIterator(List<NestedInteger> nestedList) {
stack = new Stack<>();
pushData(nestedList);
} @Override
public Integer next() {
return stack.pop().getInteger();
} @Override
public boolean hasNext() {
while(!stack.isEmpty()) {
if (stack.peek().isInteger()) {
return true;
}
pushData(stack.pop().getList());
}
return false;
} private void pushData(List<NestedInteger> nestedList) {
for (int i = nestedList.size() - 1; i >= 0; i--) {
stack.push(nestedList.get(i));
}
}
} /**
* Your NestedIterator object will be instantiated and called as such:
* NestedIterator i = new NestedIterator(nestedList);
* while (i.hasNext()) v[f()] = i.next();
*/  

Python: stack

# """
# 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.stack = []
self.list = nestedList def next(self):
"""
:rtype: int
"""
return self.stack.pop() def hasNext(self):
"""
:rtype: bool
"""
while self.list or self.stack:
if not self.stack:
self.stack.append(self.list.pop(0))
while self.stack and not self.stack[-1].isInteger():
top = self.stack.pop().getList()
for e in top[::-1]:
self.stack.append(e)
if self.stack and self.stack[-1].isInteger():
return True
return False # Your NestedIterator object will be instantiated and called as such:
# i, v = NestedIterator(nestedList), []
# while i.hasNext(): v.append(i.next())  

Python: queue

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())

C++: stack

class NestedIterator {
public:
NestedIterator(vector<NestedInteger> &nestedList) {
for (int i = nestedList.size() - 1; i >= 0; --i) {
s.push(nestedList[i]);
}
} int next() {
NestedInteger t = s.top(); s.pop();
return t.getInteger();
} bool hasNext() {
while (!s.empty()) {
NestedInteger t = s.top();
if (t.isInteger()) return true;
s.pop();
for (int i = t.getList().size() - 1; i >= 0; --i) {
s.push(t.getList()[i]);
}
}
return false;
} private:
stack<NestedInteger> s;
};

C++:deque

class NestedIterator {
public:
NestedIterator(vector<NestedInteger> &nestedList) {
for (auto a : nestedList) {
d.push_back(a);
}
} int next() {
NestedInteger t = d.front(); d.pop_front();
return t.getInteger();
} bool hasNext() {
while (!d.empty()) {
NestedInteger t = d.front();
if (t.isInteger()) return true;
d.pop_front();
for (int i = 0; i < t.getList().size(); ++i) {
d.insert(d.begin() + i, t.getList()[i]);
}
}
return false;
} private:
deque<NestedInteger> d;
};

C++: Recursion

class NestedIterator {
public:
NestedIterator(vector<NestedInteger> &nestedList) {
make_queue(nestedList);
} int next() {
int t = q.front(); q.pop();
return t;
} bool hasNext() {
return !q.empty();
} private:
queue<int> q;
void make_queue(vector<NestedInteger> &nestedList) {
for (auto a : nestedList) {
if (a.isInteger()) q.push(a.getInteger());
else make_queue(a.getList());
}
}
};

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