Medium
hasNext()很容易出错，基本上这道题flatten的操作都是在hasNext()这里完成的。我们peek()一下去看下一个要从stack里取出来的是integer还是list. 如果是integer, 那么ok没问题hasNext()return true, 而且next()直接return stack.pop.getInteger()就没事; 如果是list，说明这一坨需要flatten. 那么我们先把这一坨pop()出来，再从后到前遍历这个list加到stack里。到这里发现我们这样做不能循环call下去，只是把list转了个方向往stack里加了一下。所以我们要写一个while loop, 知道能flatten出一个integer为止；
比如[1,2,[3,[4]]]这个NestedList， 一开始我们加到stack里面是[[3,[4]], 2, 1], 然后我们call hasNext()的时候， stack.peek() = 1， 是integer, 所以直接返回true; 然后我们pop()掉1, call掉next()之后我们stack变成[[3,[4]], 2]；这时候再call hasNext(), stack.peek()会得到2， ok没问题跟1时的操作类似，最后返回true, call一下next()我们再次得到stack = [[3,[4]]], 这这时候call hasNext()的时候就会导致stack.pop(), 然后得到一个list = [3,[4]], 从后到前push到stack里得到[[4]， 3],peek()一下得到3是integer所以返回true.再call next()使得3被pop()出来，stack里面还剩[[4]], 这时候call peek得到的是list, 所以会pop出来得到list = [4], 这时候把4加到stack里面,得到stack = [4](which is different from stack = [[4]]),所以再call hasNext()的时候会检测到ni.isInteger()然后直接返回true.
/**
* // This is the interface that allows for creating nested lists.
* // You should not implement it, or speculate about its implementation
* public interface NestedInteger {
*
* // @return true if this NestedInteger holds a single integer, rather than a nested list.
* public boolean isInteger();
*
* // @return the single integer that this NestedInteger holds, if it holds a single integer
* // Return null if this NestedInteger holds a nested list
* public Integer getInteger();
*
* // @return the nested list that this NestedInteger holds, if it holds a nested list
* // Return null if this NestedInteger holds a single integer
* public List<NestedInteger> getList();
* }
*/
public class NestedIterator implements Iterator<Integer> {
Stack<NestedInteger> stack = new Stack<>();
public NestedIterator(List<NestedInteger> nestedList) {
for(int i = nestedList.size() - 1; i >= 0; i--){
stack.push(nestedList.get(i));
}
}
@Override
public Integer next() {
return stack.pop().getInteger();
}
@Override
public boolean hasNext() {
while (!stack.isEmpty()){
NestedInteger ni = stack.peek();
if (ni.isInteger()){
return true;
}
stack.pop();
List<NestedInteger> list = ni.getList();
for(int i = list.size() - 1; i >= 0; i--){
stack.push(list.get(i));
}
}
return false;
}
}
/**
* Your NestedIterator object will be instantiated and called as such:
* NestedIterator i = new NestedIterator(nestedList);
* while (i.hasNext()) v[f()] = i.next();
*/
