235. Lowest Common Ancestor of a Binary Search Tree

Posted 2020-08-09 Machelsky

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.

 

思路： Recursion。如果pq值都比root小，lca在左侧。pq值都比root大，lca在右侧。都不是，lca是root。

/**
 * 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) {
        TreeNode res;
        if(p.val<root.val&&q.val<root.val)
        {
            res=lowestCommonAncestor(root.left,p,q);
        }
        else if(p.val>root.val&&q.val>root.val)
        {
            res=lowestCommonAncestor(root.right,p,q);
        }
        else
        {
            res=root;
        }
        return res;
    }
}