题:
题目
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].
思路
题目大意
给定的一个list ,list中可能有嵌套的list,完成一个迭代器,这个迭代器返回的是 list 压平 (flatten)后的元素。
执行过程是
- NestedIterator i = new NestedIterator(nestedList);
- while (i.hasNext()) v[f()] = i.next();
即 先 hasNext() 然后 next()
解题思路
生成一个stack 存放数据。
- 初始化时,将list 中所有元素 倒序放入 stack中。
- 在hasNext()中,若 stack为空,那么返回false。若stack不为空,检查 stack的尾元素是否为 Integer 若是,那么直接返回true,否则,将尾元素变为 getList(),将这tmplist 倒序放入stack中,循环2。由于 list可能为 [[]],最后还是可能返回false。
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> tstack;
public NestedIterator(List<NestedInteger> nestedList) {
tstack = new Stack<>();
for(int i = nestedList.size()-1;i>=0;i--)
tstack.push(nestedList.get(i));
}
@Override
public Integer next() {
return tstack.pop().getInteger();
}
@Override
public boolean hasNext() {
while(!tstack.isEmpty()){
NestedInteger cur = tstack.peek();
if(cur.isInteger()) return true;
tstack.pop();
for(int i = cur.getList().size()-1;i>=0;i--) tstack.push(cur.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();
*/