lowestCommonAncestorofaBinarySearchTree.cpp

/*
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) {}
 * };
 */

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) {
    	if(root == NULL || p == NULL || q == NULL){
    		return NULL;
    	}

        int small = p -> val < q -> val ? p -> val : q -> val;
        int big = p -> val > q -> val ? p -> val : q -> val;

        TreeNode* mark = root;
        while(!(mark -> left == NULL && mark -> right == NULL)){
        	if(mark -> val > big ){
        		mark = mark -> left;
        	} else if(mark -> val < small){
        		mark = mark -> right;
        	} else{
        		return mark;
        	}
        }
        
        return mark;
    }
};