algorithm/lowestCommonAncestor.cpp at master · leobuzhi/algorithm · GitHub

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/*
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.
*/


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


/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
//better Solution
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        while((root->val-p->val)*(root->val-q->val)>0)
            root=root->val>p->val?root->left:root->right;
        return root;
    }
};