leetcode105. 從前序與中序周遊序列構造二叉樹(dfs)

目錄

• 題目來源
• 解題方法
• • DFS

題目來源

解題方法

DFS

/**
 * 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 {
    unordered_map<int,int> map;
public:
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        int n=preorder.size();
       for(int i=0;i<n;i++){
           map[inorder[i]]=i;
       }
       return helper(preorder,inorder,0,n-1,0,n-1);
    }
    TreeNode* helper(vector<int>& preorder, vector<int>& inorder, int pre_start,int pre_end,int in_start,int in_end){
        if(pre_start>pre_end)
            return NULL;
        TreeNode* root;
        root = new TreeNode(preorder[pre_start]);
        int index=map[preorder[pre_start]];
        int left_num=index-in_start;
        root->left=helper(preorder, inorder, pre_start+1,pre_start+left_num,in_start,index-1);
        root->right=helper(preorder, inorder, pre_start+left_num+1, pre_end, index+1,in_end);
        return root;
    }
};