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# 題目敘述

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].

# 解題思路

# Solution

/**
 * // 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<Integer> flattened;
    private int index;
    public NestedIterator(List<NestedInteger> nestedList) {
        flattened = new ArrayList<>();
        index = 0;
        flattened = flatten(nestedList);
    }
    private List<Integer> flatten(List<NestedInteger> nested) {
        List<Integer> result = new ArrayList<>();
        for (NestedInteger ni : nested) {
            if (ni.isInteger()) {
                result.add(ni.getInteger());
            } else {
                result.addAll(flatten(ni.getList()));
            }
        }
        return result;
    }
    @Override
    public Integer next() {
        return flattened.get(index++);
    }
    @Override
    public boolean hasNext() {
        return index < flattened.size();
    }
}
/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i = new NestedIterator(nestedList);
 * while (i.hasNext()) v[f()] = i.next();
 */




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