【LeetCode从零单刷】Lowest Common Ancestor of a Binary Search Tree

题目：

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.

解答：

树的问题，一般都会涉及到递归。对于此题，找到答案所在的分支子树，利用此函数再次递归即可。

/**
 * 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) {
        if(((p->val <= root->val) && (q->val >= root->val)) || ((p->val >= root->val) && (q->val <= root->val)))
            return root;
            
        if((p->val <= root->val) && (q->val <= root->val))
            return lowestCommonAncestor(root->left, p, q);
        else
            return lowestCommonAncestor(root->right, p, q);
    }
};