Medium
看的答案，我自己用stack的題還是做少了，特別不熟稳衬。
關(guān)鍵點：
- 整個數(shù)據(jù)結(jié)構(gòu)由一個Stack支撐
- 一開始把所有是Integer 或者nested list的所有NestedInteger都加入到stack里面去
- 根絕題意，iterate出來的都得是Integer, 所以我們hasNext()的關(guān)鍵就是要保證我們下一個pop()出來的必須得是Integer, 如果不行那就返回false. 所以我們要每次peek()一下現(xiàn)在stack里面的元素，看看是不是Integer. 如果不是衡楞，還得繼續(xù)Flatten. Flatten的過程就是把它pop()出來，然后逆序放入stack里敦姻。畫上幾筆寺酪，就可以蠻清楚地看到it works.
- next()就直接返回stack.peek().getInteger()就好了
/**
* // 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();
*/
