题目：

题解：

class Solution {
public:
    vector<int> rightSideView(TreeNode* root) {
        unordered_map<int, int> rightmostValueAtDepth;
        int max_depth = -1;

        stack<TreeNode*> nodeStack;
        stack<int> depthStack;
        nodeStack.push(root);
        depthStack.push(0);

        while (!nodeStack.empty()) {
            TreeNode* node = nodeStack.top();nodeStack.pop();
            int depth = depthStack.top();depthStack.pop();

            if (node != NULL) {
            	// 维护二叉树的最大深度
                max_depth = max(max_depth, depth);

                // 如果不存在对应深度的节点我们才插入
                if (rightmostValueAtDepth.find(depth) == rightmostValueAtDepth.end()) {
                    rightmostValueAtDepth[depth] =  node -> val;
                }

                nodeStack.push(node -> left);
                nodeStack.push(node -> right);
                depthStack.push(depth + 1);
                depthStack.push(depth + 1);
            }
        }

        vector<int> rightView;
        for (int depth = 0; depth <= max_depth; ++depth) {
            rightView.push_back(rightmostValueAtDepth[depth]);
        }

        return rightView;
    }
};