题解 | #输出二叉树的右视图#

class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     * 求二叉树的右视图
     * @param xianxu int整型vector 先序遍历
     * @param zhongxu int整型vector 中序遍历
     * @return int整型vector
     */
    unordered_map<int, int> index;
  	//利用哈希表递归建树
    TreeNode* buildTree(vector<int>& xianxu,int l1,int r1,vector<int>& zhongxu,int l2,int r2){
        if(l1>r1||l2>r2) return nullptr;
        int xianxu_root = xianxu[l1];
        int zhongxu_root = index[xianxu_root];
        TreeNode* root = new TreeNode(xianxu_root);
        int leftsize = zhongxu_root-l2;
        root->left = buildTree(xianxu, l1+1, l1+leftsize, zhongxu, l2, zhongxu_root-1);
        root->right = buildTree(xianxu, l1+leftsize+1, r1, zhongxu, zhongxu_root+1, r2);
        return root;
    }
  	//层次遍历找到最右结点
    void rightsearch(TreeNode* root,vector<int>& res){
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()){
            int s = q.size();
            for(int i = 0;i<s;i++){
                TreeNode* tmp = q.front();
                q.pop();
                if(tmp->left!=nullptr) q.push(tmp->left);
                if(tmp->right!=nullptr) q.push(tmp->right);
                if(i==s-1) res.push_back(tmp->val);
            }
        }
    }
    vector<int> solve(vector<int>& xianxu, vector<int>& zhongxu) {
        vector<int> res;
        if(xianxu.size()==0) return res;
        for(int i = 0;i<zhongxu.size();i++){
            index[zhongxu[i]] = i;
        }
        TreeNode* root = buildTree(xianxu, 0, xianxu.size()-1,zhongxu, 0, zhongxu.size()-1);
        rightsearch(root,res);
        return res;
    }
};