LeetCode 341. Flatten Nested List Iterator

原题链接在这里：https://leetcode.com/problems/flatten-nested-list-iterator/

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:
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].

题解：

Stack<NestedInteger> stk 从后向前存储nestedList的每个item. hasNext() 拿出stk顶部item, 若不是Integer就把该item从后向前放回stk中.

Note: [[]], in case of this, need to make sure the first is an integer.

Time Complexity: next, O(1). hasNext, O(n).

Space: O(n).

AC Java:

/**
 * // 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 empty list if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
public class NestedIterator implements Iterator<Integer> {
    Stack<NestedInteger> stk;
    public NestedIterator(List<NestedInteger> nestedList) {
        stk = new Stack<>();
        putListOntoStk(nestedList);
    }

    @Override
    public Integer next() {
        if(!hasNext()){
            return null;
        }

        return stk.pop().getInteger();
    }

    @Override
    public boolean hasNext() {
        while(!stk.isEmpty() && !stk.peek().isInteger()){
            List<NestedInteger> cur = stk.pop().getList();
            putListOntoStk(cur);
        }

        return !stk.isEmpty();
    }

    private void putListOntoStk(List<NestedInteger> nestedList){
        for(int i = nestedList.size() - 1; i >= 0; i--){
            stk.push(nestedList.get(i));
        }
    }
}

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i = new NestedIterator(nestedList);
 * while (i.hasNext()) v[f()] = i.next();
 */

类似Nested List Weight Sum, Zigzag Iterator, Mini Parser, Flatten 2D Vector.