LeetCode:Lowest Common Ancestor of a Binary Search Tree(BST最近公共祖先)

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.

思路：BST二叉查找树的LCA情况

查找两个node的最早的公共祖先，分三种情况：

1. 如果两个node在root的两边，那么最早的公共祖先就是root。

2. 如果两个node在root的左边，那么把root.leftChild作为root，再递归。

3. 如果两个node在root的右边，那么把root.rightChild作为root，再递归。

/**
 * 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||!q||!root) return NULL;
        
        if(max(p->val,q->val)<root->val)
            return lowestCommonAncestor(root->left,p,q);
        else if(min(p->val,q->val)>root->val)
            return lowestCommonAncestor(root->right,p,q);
        else return root;
        
    }
};