LeetCode Algorithms #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.

思路：

找到一点，它的值正好在要找的两子节点的值的中间。

如果发现当前选中的点，比两个子节点的值都大，那么要找的点在当前点的左子树中。

如果发现当前选中的点，比两个子节点的值都笑，那么要找的点在当前点的右子树中。

解：

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