235.md

235. Lowest Common Ancestor of a Binary Search Tree

Description

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.

Thinking Process

1. If p.val and q.val are both larger or smaller than root.val, they must be on the same side of the root
2. Other wise, they are on different side of root, return root

Code

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if((p.val - root.val) * (q.val - root.val) <= 0){
            return root;
        }else if(p.val < root.val){
            return lowestCommonAncestor(root.left, p, q);
        }else{
            return lowestCommonAncestor(root.right, p, q);
        }
    }
}