0199.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
  vector<int> rightSideView(TreeNode *root) {
    vector<int> ret;
    if (root == NULL) {
      return ret;
    }
    queue<TreeNode *> node_q;
    queue<int> level_q;

    node_q.push(root);
    level_q.push(0);
    ret.emplace_back(root->val);

    while (!node_q.empty()) {
      TreeNode *node = node_q.front();
      int level = level_q.front();

      if (ret.size() <= level) {
        ret.emplace_back(node->val);
      } else {
        ret[level] = node->val;
      }

      node_q.pop();
      level_q.pop();
      if (node->left) {
        node_q.push(node->left);
        level_q.push(level + 1);
      }
      if (node->right) {
        node_q.push(node->right);
        level_q.push(level + 1);
      }
    }

    return ret;
  }
};