Binary Tree Right Side View
題意：
Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.
For example: Given the following binary tree,
1 <---
/ \
2 3 <---
\ \
5 4 <---
You should return [1, 3, 4].
解題思路：
用層遍歷來走訪整個二元樹，右節點先加，每次第一個pop出來的節點便加入res中。
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public List<Integer> rightSideView(TreeNode root) {
List<Integer> res = new ArrayList<Integer>();
if (root == null) {
return res;
}
Queue<TreeNode> q = new LinkedList<TreeNode>();
q.offer(root);
while(!q.isEmpty()) {
int size = q.size();
for(int i = 0; i < size; i++) {
TreeNode top = q.poll();
if (i == 0) {
res.add(top.val);
}
if (top.right != null) {
q.offer(top.right);
}
if (top.left != null) {
q.offer(top.left);
}
}
}
return res;
}
}