Binary Tree Vertical Order Traversal
Given a binary tree, return thevertical ordertraversal of its nodes' values. (ie, from top to bottom, column by column).
If two nodes are in the same row and column, the order should be from left to right.
Example
Examples 1:
Input:
[3,9,20,null,null,15,7]
3
/\
/ \
9 20
/\
/ \
15 7
Output:
[
[9],
[3,15],
[20],
[7]
]
Examples 2:
Input:
[3,9,8,4,0,1,7]
3
/\
/ \
9 8
/\ /\
/ \/ \
4 01 7
Output:
[
[4],
[9],
[3,0,1],
[8],
[7]
]
Examples 3:
Input:
[3,9,8,4,0,1,7,null,null,null,2,5]
(0's right child is 2 and 1's left child is 5)
3
/\
/ \
9 8
/\ /\
/ \/ \
4 01 7
/\
/ \
5 2
Output:
[
[4],
[9,5],
[3,0,1],
[8,2],
[7]
]
Note
左右孩子的index分别是父节点index减一和加一
使用TreeMap是为了维持输出的顺序,Key: Column Index, Value: List
用一个column Queue去记录当前的column index
Code
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<List<Integer>> verticalOrder(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
if (root == null) {
return res;
}
TreeMap<Integer, List<Integer>> map = new TreeMap<>();
Queue<Integer> col = new LinkedList<>();
Queue<TreeNode> q = new LinkedList<>();
col.offer(0);
q.offer(root);
while (!q.isEmpty()) {
TreeNode curr = q.poll();
int column = col.poll();
map.putIfAbsent(column, new ArrayList<>());
map.get(column).add(curr.val);
if (curr.left != null) {
q.offer(curr.left);
col.offer(column - 1);
}
if (curr.right != null) {
q.offer(curr.right);
col.offer(column + 1);
}
}
for (int elem : map.keySet()) {
res.add(map.get(elem));
}
return res;
}
}
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