leetcode-235-Lowest-Common-Ancestor-of-a-Binary-Search-Tree

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描述

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).”

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     _______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.

分析

题意是找到一棵二叉搜索树中两个节点的最近公共祖先，根据二叉查找树的定义，若两节点分布在某节点的两侧，那么该节点就是公共祖先了，若都在左侧，那么公共祖先必然在该节点左侧，都在右侧是类似情况。

解决方案1(C++)

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        while(root != NULL) {
            if (p->val < root->val && q->val < root->val) {
                root = root->left;
            } else if (p->val > root->val && q->val > root->val) {
                root = root->right;
            } else {
                return root;
            }
        }
        return root;
    }
};

相关问题

(M) Lowest Common Ancestor of a Binary Tree

题目来源