Given a list, each element in the list can be a list or integer. flatten it into a simply list with integers.
If the element in the given list is a list, it can contain list too.
Example 1: Input: [[1,1],2,[1,1]] Output: [1,1,2,1,1] Explanation: flatten it into a simply list with integers. Example 2: Input: [1,2,[1,2]] Output:[1,2,1,2] Explanation: flatten it into a simply list with integers. Example 3: Input: [4,[3,[2,[1]]]] Output:[4,3,2,1] Explanation: flatten it into a simply list with integers.
思路：
递归处理，每碰到列表就递归处理，每碰到一个数字就添加进答案数组中。
复杂度O(n)
/**
* // 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) {
List<Integer> result = new ArrayList<Integer>();
for (NestedInteger ele : nestedList)
if (ele.isInteger())
result.add(ele.getInteger());
else
result.addAll(flatten(ele.getList()));
return result;
}
}
非递归
public class Solution {
// @param nestedList a list of NestedInteger
// @return a list of integer
public List<Integer> flatten(List<NestedInteger> nestedList) {
boolean isFlat = true;
List<NestedInteger> ls = nestedList;
while (isFlat) {
isFlat = false;
List<NestedInteger> newLs = new ArrayList<>();
for (NestedInteger ni : ls) {
if (ni.isInteger()) {
newLs.add(ni);
} else {
newLs.addAll(ni.getList());
isFlat = true;
}
}
ls = newLs;
}
List<Integer> r = new ArrayList<>();
for (NestedInteger ni : ls) {
r.add(ni.getInteger());
}
return r;
}
}
原文：https://www.cnblogs.com/FLAGyuri/p/12078466.html