tag 上说是 tree 类型的题目，但我觉得这更像是一个 backtracking的题目
解法：
因为题目要求所有比node.val大得值都要加上去，而这是一颗BST，所以比node.val大的值肯定都在node的右边，
base:
if is_leaf(node):
add the value of node into carry
set the value of the node = to carry
step:
if there is right node
'order is important here, you have to go to right firstto allocate all value greater than node.val' recursion(root.right,carry)
if there is left node:
since all left node is sure smaller than node.val, all we need to do here is simpliy add the number into all left node'val
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