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 allowa 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.

/**  * Definition for a binary tree node.  * public class TreeNode {  *     int val;  *     TreeNode left;  *     TreeNode right;  *     TreeNode(int x) { val = x; }  * }  */ public class Solution {     public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {         if (root.val>p.val && root.val>q.val){             return lowestCommonAncestor(root.left,p,q);         }else  if (root.val<p.val && root.val<q.val){             return lowestCommonAncestor(root.right,p,q);         }         else{             return root;         }              }  }

总结： 注意这是一棵二叉搜索树，有顺序先验。