235. Lowest Common Ancestor of a Binary Search Tree

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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.

思路就是利用二叉树的特点。

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

引申：如果不是二叉树，是普通的树应该怎么做。普通树又分有没有定义parent。没有parent的时候就是遍历，分别找到两个node的parent，再往回回溯。（理解的不是特别清楚，还需要再想想。）

http://www.cnblogs.com/springfor/p/4141924.html

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