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).”
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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。
1 /** 2 * Definition for a binary tree node. 3 * struct TreeNode { 4 * int val; 5 * TreeNode *left; 6 * TreeNode *right; 7 * TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8 * }; 9 */ 10 class Solution { 11 public: 12 TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) { 13 14 if(!p||!q||!root) return NULL; 15 16 if(max(p->val,q->val)<root->val) 17 return lowestCommonAncestor(root->left,p,q); 18 else if(min(p->val,q->val)>root->val) 19 return lowestCommonAncestor(root->right,p,q); 20 else return root; 21 22 } 23 };