leetcode [#341]

目录

• 1. 题目
• 2. 解决方案
• 3. 注意事项

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:
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].

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].

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解决方案

/** * // 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(); * } */
 * // 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> {
    private List<NestedInteger> list = new ArrayList<>();
    private Stack<Integer> num = new Stack<>();
    
    public NestedIterator(List<NestedInteger> nestedList) {
        this.list = nestedList;
        getFlatternInteger(list);
    }
    
    private void getFlatternInteger(List<NestedInteger> nest){
        for(int k = nest.size() - 1; k >= 0; k--){
            if(nest.get(k).isInteger()){
                num.push(nest.get(k).getInteger());
            } else {
                List<NestedInteger> nestList = nest.get(k).getList();
                getFlatternInteger(nestList);
            }
        }
    }

    @Override
    public Integer next() {
        return num.pop();
    }

    @Override
    public boolean hasNext() {
        return !num.isEmpty();
    }
}

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

1. 接口NestedInteger无需实现，可以直接调用相应方法。
2. 私有方法getFlatternInteger用于对输入的List nestedList做展开并存入一个堆栈。对于是数字的项，直接存入堆栈，对于是List类型的元素，递归调用该方法。
3. next()和hasNext()方法对栈做弹出和判空即可。