22.Flatten List-平面列表（容易题）

平面列表

1. 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。

2. 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]。

3. 挑战

   请用非递归方法尝试解答这道题。

4. 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));            
        }
    }
}

Last Update 2016.8.28