leetcode Construct Binary Tree from Preorder and Inorder Traversal

 

如果你没有做过根据中序遍历和后序遍历构造树，那么请先看Construct Binary Tree from Inorder and Postorder Traversal。

这题是：根据前序遍历和中序遍历构造树。

类似上一题的思路，我们可以断定正确的是中序遍历的起始下标，和前序遍历左子树的起点下标，以及右子树结束下标。根据长度一致性，来确定未知的下标即可。不难吧。

代码：　　　　　　　　　　　　　　本文地址

/**
 * 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 *fun106(vector<int> &preorder, int ps, int pe, vector<int> &inorder, int is, int ie)
    {
        if (ps > pe || is > ie) return NULL;
        TreeNode *root = new TreeNode(preorder[ps]);
        int rootval = root -> val, rootIndex;
    
        for (int i = 0; i <= ie; ++i)
        {
            if (inorder[i] == rootval)
                rootIndex = i;
        }
    
        root -> left = fun106(preorder, ps + 1, rootIndex-1-is+ps+1,inorder, is, rootIndex - 1);
        root -> right = fun106(preorder, pe-(ie-(rootIndex+1)), pe, inorder, rootIndex + 1, ie);
    
        return root;
    }
    TreeNode *buildTree(vector<int> &preorder, vector<int> &inorder)
    {
        if (preorder.size() == 0) return NULL;
        return fun106(preorder, 0, preorder.size() - 1, inorder, 0, inorder.size() - 1);
    }
};