Google/LintCode：M-摊平嵌套的列表

题目

题目来源：Link

给你一个嵌套的列表，实现一个迭代器将其摊平。

一个列表的每个元素可能是整数或者一个列表。


 注意事项

You don't need to implement the remove method.

您在真实的面试中是否遇到过这个题？ 

Yes

样例

给出列表 

[[1,1],2,[1,1]]

，经过迭代器之后返回 

[1,1,2,1,1]

。
给出列表 

[1,[4,[6]]]

，经过迭代器之后返回 

[1,4,6]

。

代码

（1）巧妙利用堆栈（用List实现）

/**
* // 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();
* }
*/
import java.util.Iterator;

public class NestedIterator implements Iterator<Integer> {

List<NestedInteger> list;
public NestedIterator(List<NestedInteger> nestedList) {
// Initialize your data structure here.
if(nestedList==null) return;

list = nestedList;
}

// @return {int} the next element in the iteration
@Override
public Integer next() {
// Write your code here
return nextNum;
}

Integer nextNum;
// @return {boolean} true if the iteration has more element or false
@Override
public boolean hasNext() {
// Write your code here
while(list.size()!=0){
NestedInteger tmp = list.get(0);
list.remove(0);

if(tmp.isInteger()){
//list.remove(0);
//list.add(0,tmp);
nextNum = tmp.getInteger();
return true;
}else{
List<NestedInteger> tmpList = tmp.getList();
if(tmpList.size()!=0){
for(int i=tmpList.size()-1; i>=0; i--){
list.add(0, tmpList.get(i));
}
}
}
}

return false;
}

@Override
public void remove() {
if(hasNext()){
list.remove(0);
}
}
}

/**
* Your NestedIterator object will be instantiated and called as such:
* NestedIterator i = new NestedIterator(nestedList);
* while (i.hasNext()) v.add(i.next());
*/

（2）迭代（会超时）

/**
* // 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();
* }
*/
import java.util.Iterator;

public class NestedIterator implements Iterator<Integer> {

List<Integer> list = new ArrayList<Integer>();
public NestedIterator(List<NestedInteger> nestedList) {
// Initialize your data structure here.
if(nestedList==null) return;

solve(nestedList);
}
void solve(List<NestedInteger> tList){
if(tList.size()==0) return;

for(int i=0; i<tList.size(); i++){
NestedInteger tmp = tList.get(i);
if(tmp.isInteger()){
list.add(tmp.getInteger());
}else{
solve(tmp.getList());
}
}
}
// @return {int} the next element in the iteration
@Override
public Integer next() {
// Write your code here
int tmp = list.get(0);
list.remove(0);//一定要remove，否则报错
return tmp;
}

// @return {boolean} true if the iteration has more element or false
@Override
public boolean hasNext() {
// Write your code here
if(list.size()>0)
return true;
else
return false;
}

@Override
public void remove() {
if(hasNext()){
list.remove(0);
}
}
}

/**
* Your NestedIterator object will be instantiated and called as such:
* NestedIterator i = new NestedIterator(nestedList);
* while (i.hasNext()) v.add(i.next());
*/