235_Lowest Common Ancestor of a Binary Search Tree

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.

给定一个二叉搜索树，找到两个节点的最小祖先

由于二叉搜索树性质：一个节点的值大于左边所有节点，小于右边所有节点，遍历该二叉搜索树：

1. 根节点的值大于两个节点中大的，则继续向右侧节点遍历

2. 根节点值小于左侧节点中小的，继续向左侧节点遍历

3. 一旦出现根节点值等于两个值其中一个，或者根节点的值大于小值，小于大值，则根节点为所得结果

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
struct TreeNode* lowestCommonAncestor(struct TreeNode* root, struct TreeNode* p, struct TreeNode* q) {
    struct TreeNode* result = NULL;
    struct TreeNode* t = NULL;
    struct TreeNode* big = NULL;
    struct TreeNode* small = NULL;
    
    t = root;
    if(p->val >= q->val)
    {
        big = p;
        small = q;
    }
    else
    {
        big = q;
        small = p;
    }
    while(t != NULL)
    {
        if(t->val > big->val)
        {
            t = t->left;
            continue;
        }
        if(t->val < small->val)
        {
            t = t->right;
            continue;
        }
        if(t->val == big->val || t->val == small->val || (t->val >= small->val && t->val <= big->val))
        {
            result = t;
            break;
        }
    }
    return result;
}