30 Day Challenge Day 4 | Hackrank

题解

俯视角度下观察结果，我想到用root作为基准，令其序号为0，相应的，左节点在根节点基础上序号减一，而右节点加一。

用一个队列，自上而下逐层遍历。因为观察角度是从上往下，上层的节点会遮住同一列的下层所有节点。一边往下，一遍往左右两侧探索，遇到水平方向更远的节点都要保存到结果中。

    void topView(Node * root) {
        // Take root position as a base, move left -1, move right +1
        if(!root) return;

        vector<int> leftward, rightward;

        queue<pair<Node*, int>> q;
        q.push({root, 0});

        while(!q.empty()) {
            int size = q.size();
            for(int i = 0; i < size; i++) {
                pair<Node*, int> n = q.front();
                q.pop();
                if(n.second < 0 && leftward.size() < abs(n.second)) {
                    leftward.push_back(n.first->data);
                } else if(n.second > 0 && rightward.size() < abs(n.second)) {
                    rightward.push_back(n.first->data);
                }
                if(n.first->left) {
                    q.push({n.first->left, n.second - 1});
                }
                if(n.first->right) {
                    q.push({n.first->right, n.second + 1});
                }
            }
        }

        for(int i = leftward.size()-1; i >= 0; i--) {
            cout << leftward[i] << " ";
        }
        cout << root->data << " ";
        for(int i = 0; i < rightward.size(); i++) {
            cout << rightward[i] << " ";
        }

        return;
    }