技術標籤：leetcode刷題

根據前序和中序建樹

class Solution {
public:
    unordered_map<int,int> pos;
    TreeNode* build(vector<int>& preorder, vector<int>& inorder,int il,int ir,int pl,int pr){
        if(il > ir) return nullptr;
        TreeNode* root = new TreeNode(preorder[pl]);
        int k = pos[root->val];
        root->left = build(preorder,inorder,il,k-1,pl+1,pl+k-il);
        root->right = build(preorder,inorder,k+1,ir,pl+k-il+1,pr);
        return root;
    }
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        int n = inorder.size();
        for(int i = 0; i < inorder.size();++i){
            pos[inorder[i]] = i;
        }
        return build(preorder,inorder,0,n-1,0,n-1);
    }
};
 

根據中序和後序建樹

class Solution {
public:
    vector<int> sinorder,spostorder;
    unordered_map<int,int> pos;
    TreeNode* build(int il,int ir,int pl,int pr){
        if(il > ir) return nullptr;
        TreeNode* root = new TreeNode(spostorder[pr]);
        int k = pos[root->val];
        root->left = build(il,k-1,pl,pl+(k-1-il));
        root->right = build(k+1,ir,pl+(k-1-il)+1,pr-1); 
        return root;
    }
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        int n = inorder.size();
        sinorder = inorder;
        spostorder = postorder; //賦值
        for(int i = 0; i < inorder.size(); ++i){
            pos[inorder[i]] = i;   //中序陣列定位
        }
        return build(0,n-1,0,n-1);
    }
};