Number of Airplanes in the Sky
把起飞和降落看成括号就好理解了。比如(())()，左括号就是起飞（1），又括号就是降落（-1）。从左到右挨着遍历，算出遍历中的最大值。实现的难点在于comparator, 如果起飞降落在同一时间则可以用 return p1.flag-p2.flag表示。结果小于零则p1在前，大于零则p2在前。
776. Split BST
Problem: Given a Binary Search Tree (BST) with root node root , and a target value V , split the tree into two subtrees where one subtree has nodes that are all smaller or equal to the target value, while the other subtree has all nodes that are greater than the target value. It's not necessarily the case that the tree contains a node with value V . Additionally, most of the structure of the original tree should remain. Formally, for any child C with parent P in the original tree, if they are both in the same subtree after the split, then node C should still have the parent P. You should output the root TreeNode of both subtrees after splitting, in any order. Example 1: Input: root = [4,2,6,1,3,5,7], V = 2 Output: [[2,1],[4,3,6,null,null,5,7]] Explanation: Note that root, output[0], and output[1] are TreeNode objects, not arrays. The given tree [4,2,6,1,3,5,7] is represented by the following diagram: 4 / \ 2 6 / \ / \ 1
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