Leetcode: Binary Tree Level Order Transversal II
Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root). For example:
Given binary tree {3,9,20,#,#,15,7},
3
/ \
9 20
/ \
15 7
return its bottom-up level order traversal as:
[
[15,7],
[9,20],
[3]
]
第二遍方法:
这道题在groupon面经里面有,有一个follow up 是能不能右对齐输出。那就在29行记录每一行的最大size,然后在输出的时候根据最大size补齐空格
/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> res = new ArrayList<List<Integer>>();
if (root == null) return res;
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
while (!queue.isEmpty()) {
List<Integer> item = new ArrayList<Integer>();
int size = queue.size();
for (int i=0; i<size; i++) {
TreeNode cur = queue.poll();
item.add(cur.val);
if (cur.left != null) {
queue.add(cur.left);
}
if (cur.right != null) {
queue.add(cur.right);
}
}
res.add(0, new ArrayList<Integer>(item));
}
return res;
}
}
在Binary Tree Level Order Transversal的基础上难度:20,只需要对最后结果做一个倒序就好。格式是Collections.reverse(List<?> list)
/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
ArrayList<ArrayList<Integer>> lists = new ArrayList<ArrayList<Integer>> ();
if (root == null) return lists;
LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
queue.add(root);
int ParentNumInQ = 1;
int ChildNumInQ = 0;
ArrayList<Integer> list = new ArrayList<Integer>();
while (!queue.isEmpty()) {
TreeNode cur = queue.poll();
list.add(cur.val);
ParentNumInQ--;
if (cur.left != null) {
queue.add(cur.left);
ChildNumInQ++;
}
if (cur.right != null) {
queue.add(cur.right);
ChildNumInQ++;
}
if (ParentNumInQ == 0) {
ParentNumInQ = ChildNumInQ;
ChildNumInQ = 0;
lists.add(list);
list = new ArrayList<Integer>();
}
}
Collections.reverse(lists);
return lists;
}
}
注意38行的写法,Collections.reverse()跟Arrays.sort()函数一样,都是void返回型,然后改变作用在argument上
还有if (root == null) return null;
ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();这样会出错:
Input:{}Output:nullExpected:[]
DFS 做法:
public class Solution {
public List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> wrapList = new LinkedList<List<Integer>>();
levelMaker(wrapList, root, 0);
return wrapList;
} public void levelMaker(List<List<Integer>> list, TreeNode root, int level) {
if(root == null) return;
if(level >= list.size()) {
list.add(0, new LinkedList<Integer>());
}
levelMaker(list, root.left, level+1);
levelMaker(list, root.right, level+1);
list.get(list.size()-level-1).add(root.val);
}
}
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