Construct Binary Tree from Preorder and Inorder Traversal

Posted 2021-01-21 ruruozhenhao

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

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

For example, given

preorder = [3,9,20,15,7]
inorder = [9,3,15,20,7]

Return the following binary tree:

    3
   /   9  20
    /     15   7

 

Approach #1: C++.

/**
 * 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 {
public:
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        return solve(preorder, 0, inorder, 0, inorder.size()-1);
    }
    
private:
    TreeNode* solve(vector<int>& preorder, int ps, vector<int>& inorder, int is, int ie) {
        if (ps > preorder.size() || is > ie) return NULL;
        TreeNode* root = new TreeNode(preorder[ps]);
        int inIndex = 0;
        for (int i = is; i <= ie; ++i)
            if (inorder[i] == root->val) 
                inIndex = i;
        TreeNode* leftchild = solve(preorder, ps+1, inorder, is, inIndex-1);
        TreeNode* rightchild = solve(preorder, ps+inIndex-is+1, inorder, inIndex+1, ie);
        root->left = leftchild;
        root->right = rightchild;
        return root;
    }
};

　　

Approach #2: Java.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        return helper(0, 0, inorder.length - 1, preorder, inorder);
    }
    private TreeNode helper(int preStart, int inStart, int inEnd, int[] preorder, int[] inorder) {
        if (preStart > preorder.length - 1 || inStart > inEnd) return null;
        TreeNode root = new TreeNode(preorder[preStart]);
        int inIndex = 0;
        for (int i = inStart; i <= inEnd; ++i) {
            if (inorder[i] == root.val)
                inIndex = i;
        }
        root.left = helper(preStart + 1, inStart, inIndex - 1, preorder, inorder);
        root.right = helper(preStart + inIndex - inStart + 1, inIndex + 1, inEnd, preorder, inorder);
        return root;
    }
        
}

　　

Approach #3: Python.

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def buildTree(self, preorder, inorder):
        """
        :type preorder: List[int]
        :type inorder: List[int]
        :rtype: TreeNode
        """
        if inorder:
            ind = inorder.index(preorder.pop(0))
            root = TreeNode(inorder[ind])
            root.left = self.buildTree(preorder, inorder[0:ind])
            root.right = self.buildTree(preorder, inorder[ind+1:])
            return root