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## Problem

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:

Input: [[1,1],2,[1,1]]
Output: [1,1,2,1,1]
Explanation: By calling next repeatedly until hasNext returns false,
the order of elements returned by next should be: [1,1,2,1,1].


Example 2:

Input: [1,[4,[6]]]
Output: [1,4,6]
Explanation: By calling next repeatedly until hasNext returns false,
the order of elements returned by next should be: [1,4,6].


## Code

/**
* // 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;
public NestedIterator(List<NestedInteger> nestedList) {
stack = new Stack<>();
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() {
// 注意是while，因为有可能nested中包含了nested，需要把所有的nested解开为integer
while(!stack.isEmpty()){
NestedInteger curr = stack.peek();
if(curr.isInteger()){
return true;
} else {
stack.pop();
for(int i = curr.getList().size() - 1; i >= 0; i--){
NestedInteger temp = curr.getList().get(i);
stack.push(temp);
}
}
}

return false;
}
}