二叉树的还原（前序和中序中序和后序）

Posted 2021-07-18 Moua

根据前序和中序序列还原二叉树

class Solution {
public:
    TreeNode* _buildTree(vector<int>& preorder,vector<int>& inorder,int& prevIndex,int inBegin,int inEnd)
    {
        if(prevIndex >= preorder.size())
            return nullptr;
        //中序序列不存在
        if(inEnd < inBegin)
            return nullptr;
        //将中序序列分割成左子树和右子树
        int inRoot = inBegin;
        for(int i = inBegin;i <= inEnd;i++)
        {
            if(inorder[i] == preorder[prevIndex])
            {
                //根节点在中序序列中的位置
                inRoot = i;
                break;
            }
        }
        //创建根节点
        TreeNode* root = new TreeNode(preorder[prevIndex]);
        prevIndex++;
        //左子树
        root->left = _buildTree(preorder,inorder,prevIndex,inBegin,inRoot-1);
        //右子树
        root->right = _buildTree(preorder,inorder,prevIndex,inRoot+1,inEnd);

        return root;
    }
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        int prevIndex = 0;
        return _buildTree(preorder,inorder,prevIndex,0,inorder.size()-1);
    }
};

根据中序和后序序列还原二叉树

class Solution {
public:
    TreeNode* _buildTree(vector<int>& inorder,vector<int>& postorder,int& postIndex,int inBegin,int inEnd)
    {
        if(postIndex < 0)
            return nullptr;
        
        if(inBegin > inEnd)
            return nullptr;
        //后序序列从后向前遍历依次为根->右孩子->左孩子
        //根据后序序列中的根节点在中序序列中找到位置下标
        int inIndex = inBegin;
        for(int i = inBegin;i <= inEnd;i++)
        {
            if(postorder[postIndex] == inorder[i])
            {
                inIndex = i;
                break;
            }
        }
        //创建根节点
        TreeNode* root = new TreeNode(postorder[postIndex]);
        postIndex--;
        //创建左子树
        root->right = _buildTree(inorder,postorder,postIndex,inIndex+1,inEnd);
        root->left = _buildTree(inorder,postorder,postIndex,inBegin,inIndex-1);

        return root;
    }
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        int postIndex = postorder.size()-1;
        return _buildTree(inorder,postorder,postIndex,0,inorder.size()-1);        
    }
};