題：

題目

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

題目大意

給定的一個list ,list中可能有巢狀的list，完成一個迭代器，這個迭代器返回的是 list 壓平 (flatten)後的元素。

執行過程是

• NestedIterator i = new NestedIterator(nestedList);
• while (i.hasNext()) v[f()] = i.next(); 即 先 hasNext() 然後 next()

解題思路

生成一個stack 存放資料。

1. 初始化時，將list 中所有元素 倒序放入 stack中。
2. 在hasNext()中，若 stack為空，那麼返回false。若stack不為空，檢查 stack的尾元素是否為 Integer 若是，那麼直接返回true，否則，將尾元素變為 getList()，將這tmplist 倒序放入stack中，迴圈2。由於 list可能為 [[]]，最後還是可能返回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();
 */