341. Flatten Nested List Iterator

难度: Medium

题目描述

You are given a nested list of integers nestedList. Each element is either an integer or a list whose elements may also be integers or other lists. Implement an iterator to flatten it.

Implement the NestedIterator class:

• NestedIterator(List<NestedInteger> nestedList) Initializes the iterator with the nested list nestedList.
• int next() Returns the next integer in the nested list.
• boolean hasNext() Returns true if there are still some integers in the nested list and false otherwise.

Your code will be tested with the following pseudocode:

initialize iterator with nestedList
res = []
while iterator.hasNext()
    append iterator.next() to the end of res
return res

If res matches the expected flattened list, then your code will be judged as correct.

 

Example 1:

Input: nestedList = [[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: nestedList = [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].

 

Constraints:

• 1 <= nestedList.length <= 500
• The values of the integers in the nested list is in the range [-106, 106].

解题思路

代码实现

解决方案

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> {

    private List<NestedInteger> nestedList;
    private LinkedList<Integer> stack;
    private int curIndex = -1;

    public NestedIterator(List<NestedInteger> nestedList) {
        this.nestedList = nestedList;
        stack = new LinkedList<>();
    }

    @Override
    public Integer next() {
        if (curIndex == -1) {
            for (int i = nestedList.size() - 1; i > -1; i--) {
                flat(nestedList.get(i));
            }
            curIndex++;
        }
        if (stack.size() > 0) {
            return stack.pop();
        }
        return stack.pop();

    }

    private void flat(NestedInteger cur) {
        if (cur == null) {
            return;
        }
        if (cur.isInteger()) {
            stack.push(cur.getInteger());
        } else {
            List<NestedInteger> list = cur.getList();
            for (int i = list.size() - 1; i > -1; i--) {
                flat(list.get(i));
            }
        }
    }

    @Override
    public boolean hasNext() {
        if (curIndex == -1) {
            for (int i = nestedList.size() - 1; i > -1; i--) {
                flat(nestedList.get(i));
            }
            curIndex++;
        }
        return !stack.isEmpty();
    }
}

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