LeetCode 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.

分析：

在二叉查找树种，寻找两个节点的最低公共祖先。

1、假设a、b都比根节点小，则在左子树中递归查找公共节点。

2、假设a、b都比根节点大。则在右子树中查找公共祖先节点。

3、假设a、b一个比根节点大。一个比根节点小，或者有一个等于根节点，则根节点即为最低公共祖先。

class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
            if (root == NULL) {
            return NULL;
        }
        int min = (p->val > q->val ? q->val:p->val);
        int max = (p->val > q->val ? p->val: q->val);
        if(max < root->val)
            return lowestCommonAncestor(root->left, p, q);
        if (min > root->val) {
            return lowestCommonAncestor(root->right, p, q);
        }
        return root;
    }
};