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].
非常精巧地使用stack。push all the nestedList into the stack from back to front，so when we pop the stack, it returns the very first element
执行hasNext()的时候，如果peek()是integer, return true；否则是一个List<NestedInteger>, 则flatten这个list，同样的，把list里的NestedInteger倒序push入栈
/**
* // 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> st; public NestedIterator(List<NestedInteger> nestedList) {
st = new Stack<NestedInteger>();
for (int i=nestedList.size()-1; i>=0; i--) {
st.push(nestedList.get(i));
}
} @Override
public Integer next() {
return st.pop().getInteger();
} @Override
public boolean hasNext() {
while (!st.isEmpty()) {
if (st.peek().isInteger()) return true;
List<NestedInteger> cur = st.pop().getList();
for (int i=cur.size()-1; i>=0; i--) {
st.push(cur.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();
*/