235. Lowest Common Ancestor of a Binary Search Tree 二叉搜索树的LCA

Posted 2020-09-24 Long Long Journey

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.

二叉搜索树，公共祖先的值一定大于等于较小的节点，小于等于较大的节点。换言之，在遍历树的时候，如果当前结点大于两个节点，则结果在当前结点的左子树里，如果当前结点小于两个节点，则结果在当前节点的右子树里。

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

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