Longest Increasing Subsequence
一开始想用O(n)来做，结果是错的。要用两层循环DP来做。假设当前index 为i，在0<j<i-1中。如果nums[i]>nums[j]， dp[i]=max(dp[j])+1。dp[i]很不好理解，其实dp[n-1]并不是所求的结果，而是以nums[i]为最大值，的longest increasing subsequence。看了2个小时，终于把这一点看懂了。leetcode article 没说清楚啊。
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 / \ ...
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