22.Flatten List-平面列表（容易题）

最新推荐文章于 2021-06-27 15:19:10 发布

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#LintCode #递归 #非递归

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本文介绍了一种将嵌套列表转换为单一整数列表的方法，包括递归与非递归两种实现方式。递归方法直观易懂，而非递归方法使用栈结构进行迭代处理。

平面列表

题目

给定一个列表，该列表中的每个要素要么是个列表，要么是整数。将其变成一个只包含整数的简单列表。

注意事项

如果给定的列表中的要素本身也是一个列表，那么它也可以包含列表。
样例

给定 [1,2,[1,2]]，返回 [1,2,1,2]。
给定 [4,[3,[2,[1]]]]，返回 [4,3,2,1]。
挑战

请用非递归方法尝试解答这道题。
题解

1.递归解法

/**
 * // 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 Solution {

    // @param nestedList a list of NestedInteger
    // @return a list of integer
    List<Integer> result = new ArrayList<Integer>();

    public List<Integer> flatten(List<NestedInteger> nestedList) {
        for (int i=0;i<nestedList.size();i++)
        {
            if (nestedList.get(i).isInteger())
            {
                result.add(nestedList.get(i).getInteger());
            }
            else
            {
                flatten(nestedList.get(i).getList());
            }
        }

        return result;
    }
}

2.非递归解法

/**
 * // 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 Solution {

    // @param nestedList a list of NestedInteger
    // @return a list of integer

    public List<Integer> flatten(List<NestedInteger> nestedList) {
        Stack<NestedInteger> stack = new Stack<>();
        List<Integer> result = new ArrayList<Integer>();
        pushInStack(stack,nestedList);
        while (!stack.empty())
        {
            if (stack.peek().isInteger())
            {
                result.add(stack.pop().getInteger());
            }
            else
            {
                pushInStack(stack,stack.pop().getList());
            }

        }
        Collections.reverse(result);
        return result;
    }

    private void pushInStack(Stack<NestedInteger> stack,List<NestedInteger> nestedList)
    {
        for (int i=0;i<nestedList.size();i++)
        {
            stack.push(nestedList.get(i));            
        }
    }
}