LeetCode 235----Lowest Common Ancestor of a Binary Search

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.

解题思路：

1. 找到结点p，一层一层向上查找，查找右子树是否包含q；
2. 若右子树不包含q，向上一层移动；
3. 若p、q均找到，则返回root；
4. 若root等于p或q，则root即为LCA

若root等于p或q，则root即为LCA，如图：

寻找LCA图示如下：

/** * 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(root==NULL)            return NULL;        if(root==p||root==q)            return root;        TreeNode* left=lowestCommonAncestor(root->left,p,q);        TreeNode* right=lowestCommonAncestor(root->right,p,q);        if(left&&!right)    //存在左结点            return left;        if(!left&&right)    //存在右结点            return right;        if(left&&right)            return root;    }};