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 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) {

　　　　//make sure p.val < q.val
　　　　if (p.val > q.val) {
　　　　　　TreeNode tmp = p;
　　　　　　p = q;
　　　　　　q = tmp;
　　　　}
　　　　if (root.val == p.val || root.val == q.val) {
　　　　　　return root;
　　　　}
　　　　if (root.val > p.val && root.val < q.val) {
　　　　　　return root;
　　　　}
　　　　if (root.val < p.val) {
　　　　　　return lowestCommonAncestor(root.right, p, q);
　　　　}
　　　　return lowestCommonAncestor(root.left, p, q);
　　}
}