1038. Binary Search Tree to Greater Sum Tree

• DFS

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
private:
    int sum = 0;
public:
    TreeNode* bstToGst(TreeNode* root)
    {
        dfs(root);
        return root;
    }

    void dfs(TreeNode* root)
    {
        if (!root) return;
        dfs(root->right);
        root->val += sum;
        sum = root->val;
        dfs(root->left);
    }
};

• T: $O(n)$

• S: $O(n)$

• BFS

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* bstToGst(TreeNode* root)
    {
        int sum = 0;
        stack<TreeNode*> stk;
        TreeNode* node = root;
        while (node || !stk.empty())
        {
            while (node)
            {
                stk.push(node);
                node = node->right;
            }
            node = stk.top(); stk.pop();
            sum += node->val;
            node->val = sum;
            node = node->left;
        }
        return root;
    }
};

• T: $O(n)$
• S: $O(n)$