235. Lowest Common Ancestor of a Binary Search Tree

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 2and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

思路：因为是bst, 如果root，的值介于pq之间，root就是共同祖先。

root不是共同祖先，则递归地判断左右子树是不是公共祖先。

递归版：

1 class Solution {
2     public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
3         if((root.val-p.val)*(root.val-q.val)<=0) return root;
4         return lowestCommonAncestor((root.val-p.val)>0?root.left:root.right, p, q);
5     }
6 }

循环版：

1 class Solution {
2     public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
3         while((root.val-p.val)*(root.val-q.val)>0)
4             root = ((root.val-p.val)>0?root.left:root.right);
5         return root;
6     
7     }
8 }