05 August 2008
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
[[1,1],2,[1,1]]
By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: 1
[1,1,2,1,1]
Example 2:
Given the list 1
[1,[4,[6]]]
By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: 1
[1,4,6]
建立压平嵌套链表的迭代器，而迭代器一般都是用迭代的方法来解，如果要用递归一般都需用栈来辅助遍历，由于栈的后进先出的特性，在对向量遍历的时候，从后往前把对象压入栈中，那么第一个对象最后压入栈就会第一个取出来处理，我们的hasNext()函数需要遍历栈，并进行处理，如果栈顶元素是整数，直接返回true，如果不是，那么移除栈顶元素，并开始遍历这个取出的list，还是从后往前压入栈，循环停止条件是栈为空，返回false。
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/**
* // 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) {
//构造器先把所有NestedIterator的对象从后向前加入到栈中
for (int i = nestedList.size() - 1; i >= 0; i--) {
stack.push(nestedList.get(i));
}
}
@Override
public Integer next() {
// 返回stack顶部元素的整数
return stack.pop().getInteger();
}
@Override
public boolean hasNext() {
while (!stack.isEmpty()) {
NestedInteger ni = stack.peek();
if (ni.isInteger()) { // 如果时数字，返回
return true;
}
stack.pop(); // 否则是数组，嵌套从尾到头压入栈
for (int i = ni.getList().size() - 1; i >= 0; i--) {
stack.push(ni.getList().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();
*/