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

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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.

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問題描述

給定二叉排序樹， 找出兩個節點的最小公共祖先

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問題分析

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解法1

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);
        else if(root .val < p.val && root.val < q.val) return lowestCommonAncestor(root.right, p, q);
        else return root;
    }
}

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解法2

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        TreeNode tempRoot = root;

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

        return tempRoot;
    }
}