199. Binary Tree Right Side View

Given the root of 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.

BFS搜索。层级遍历各个节点，当节点为该层级最后一个节点时，将其值加入返回列表。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> ret = new ArrayList<>();
        if(root == null) return ret;
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        int size = 1;
        while(!q.isEmpty()){
            for(int i = 0; i < size; i++){
                TreeNode curr = q.poll();
                if(i == size - 1) ret.add(curr.val);
                if(curr.left != null) q.add(curr.left);
                if(curr.right != null) q.add(curr.right);
            }
            size = q.size();
        }
        return ret;
    }
}