【leetcode】Construct Binary Tree from Inorder and Postorder Traversal

Question： 

Given inorder and postorder traversal of a tree, construct the binary tree.

Note:
You may assume that duplicates do not exist in the tree.

Anwser 1 ：   

/**  * Definition for binary tree  * struct TreeNode {  *     int val;  *     TreeNode *left;  *     TreeNode *right;  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}  * };  */ class Solution { public:     TreeNode *createTree(vector<int> &inorder, int inBeg, int inEnd, vector<int> &postorder, int postBeg, int postEnd)     {         if (inBeg > inEnd)  return NULL;                      int root = postorder[postEnd];                  int index;                  for(int i = inBeg; i <= inEnd; i++)             if (inorder[i] == root)             {                 index = i;                 break;             }                      int len = index - inBeg;         TreeNode *left = createTree(inorder, inBeg, index - 1, postorder, postBeg, postBeg + len - 1);         TreeNode *right = createTree(inorder, index + 1, inEnd, postorder, postBeg + len, postEnd - 1);                  TreeNode *node = new TreeNode(root);         node->left = left;         node->right = right;                  return node;     }          TreeNode *buildTree(vector<int> &inorder, vector<int> &postorder) {         // Start typing your C/C++ solution below         // DO NOT write int main() function         if (inorder.size() == 0) return NULL;                      return createTree(inorder, 0, inorder.size() - 1, postorder, 0, postorder.size() - 1);     } };

Anwser 2 ：    

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    std::unordered_map<int,int> index_map;    void buildMap(vector<int> &inorder) {        index_map.clear();        for (int i = 0; i < inorder.size(); i++) {            index_map[inorder[i]] = i;        }    }    TreeNode *buildTreeInOrder(vector<int> &postorder, int post_offset, vector<int> &inorder, int in_offset, int len)     {        if (!len)  return NULL;        TreeNode *root = new TreeNode(postorder[post_offset]);        int i = index_map[postorder[post_offset]];        root->right = buildTreeInOrder(postorder, post_offset-1, inorder, in_offset, in_offset-i);        root->left = buildTreeInOrder(postorder, post_offset-(in_offset-i)-1, inorder, in_offset-(in_offset-i)-1, len-(in_offset-i)-1);        return root;    }    TreeNode *buildTree(vector<int> &inorder, vector<int> &postorder) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if (postorder.size() == 0 && inorder.size() == 0) {            return NULL;        }        buildMap(inorder);        return buildTreeInOrder(postorder, postorder.size()-1, inorder, inorder.size()-1, postorder.size());    }};

參考推薦：

Construct Binary Tree from Inorder and Postorder Traversal