Leetcode | Lowest Common Ancestor of a Binary Search Tree

question

Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

      _______6______     /              \  ___2__           ___8__ /      \         /      \ 0      _4        7       9       /  \      3   5

For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

explaination

节点p、q和根节点的关系有三种情况

（1）p、q在根节点的左结点

（2）p、q在根节点的右结点

（3）p、q分别在根节点的左右结点

由于是二叉查找树，所以在左结点比根结点小 右结点比根结点大，可以根据这个性质进行判断

solution

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {    while((root.val-p.val)*(root.val-q.val) > 0)    {        root= root.val>p.val?root.left : root.right;    }    return root;}