LeetCode:Convert BST to Greater Tree

538. Convert BST to Greater Tree

Given a Binary Search Tree (BST), convert it to a Greater Tree such that every key of the original BST is changed to the original key plus sum of all keys greater than the original key in BST.

Example:

Input: The root of a Binary Search Tree like this:
              5
            /   \
           2     13

Output: The root of a Greater Tree like this:
             18
            /   \
          20     13

思路：注意利用BST中序遍历有序的特点，逆序遍历，在加和的时候一定要注意，先遍历右子树，处理当前节点，再遍历左子树。

void addGreaterSum(TreeNode*t, int* sum,bool* isfirst)
{
    if (t == NULL)
        return;
    addGreaterSum(t->right, sum, isfirst);
    if (t->right == NULL&&*isfirst)
    {
        *isfirst = false;
        *sum = t->val;
    }
    else
    {
        int temp = t->val;
        t->val += *sum;
        *sum += temp;
    }

    addGreaterSum(t->left, sum, isfirst);
}
TreeNode* convertBST(TreeNode* root) 
{
    int sum = 0;
    bool isfirst = true;
    addGreaterSum(root, &sum, &isfirst);
    return root;
}

 

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