235. Lowest Common Ancestor of a Binary Search Tree java solutions

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.

 

Subscribe to see which companies asked this question

p和q的LCA是恰好把p、q分叉的节点，也就是LCA的值介于p、q之间

从最高层的祖先root开始往下，

若不满足LCA条件，往p、q所在子树递归。

/**
 * 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((p.val - root.val)*(root.val - q.val) >= 0) return root;
        return root.val > p.val ? lowestCommonAncestor(root.left,p,q) : lowestCommonAncestor(root.right,p,q);
    }
}

解法二 非递归：

/**
 * 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) {
        while(root != null){
            if(root.val > p.val && root.val > q.val) root = root.left;
            else if(root.val < p.val && root.val < q.val) root = root.right;
            else return root;
        }
        return root;
    }
}