leetcode-235-Lowest Common Ancestor of a Binary Search Tree

标签：leetcode

                         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.

BST中，给出两个节点，求离它们最近的祖先。

因为是BST，root的左子树的值都小于root，root的右子树的值都大于root。

两个节点的公共路径的最后一个节点，即为最近的祖先。

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* solve(TreeNode* root,TreeNode* p,TreeNode* q){
        if (p->val < root->val && q->val > root->val || p->val > root->val && q->val < root->val) return root; 
        if (p->val == root->val || q->val == root->val) return root;
        if (p->val < root->val && q->val < root->val) return solve(root->left,p,q);
        if (p->val > root->val && q->val > root->val) return solve(root->right,p,q);
    }
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        return solve(root,p,q);
    }
};

非递归

/**
 * 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) {
        while (root) {
            if (p->val < root->val && q->val < root->val) root = root->left; // 向左
            else if(p->val > root->val && q->val > root->val) root = root->right; //向右
            else return root;
        }
    }
};

leetcode-235-Lowest Common Ancestor of a Binary Search Tree

标签：leetcode

原文地址：http://blog.csdn.net/u014705854/article/details/46915681