LeetCode235: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.
二叉搜索樹的最低公共祖先節點。
因為二叉搜索樹的特性，即不論什麽一個節點的左子樹中的節點值都比該節點值小。不論什麽一個節點的右子樹中的節點值都比該節點值大。能夠依據這個特性在二叉樹中查找。
對於給定的兩個節點指針p和q，先調整p和q,使p指向值較小的那個節點。q指向值較大的那個節點；
然後從根節點node開始遍歷，假設q的值小於node的值，表示p和q都在node的左子樹中，更新node為node->left。
假設p的值大於node的值。表示p和q都在node的右子樹中，更新node為node->right；
否則表示找到的這個節點就是p和q的最低公共祖先節點。
runtime:40ms
/**
* 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;
//使p保存值較小的節點。q保存值較大的節點
if(p->val > q->val)
{
/*
TreeNode *tmp=p;
p=q;
q=tmp;
*/
//上面的代碼能夠直接寫成以下的樣子
swap(p,q);
}
TreeNode *result=root;
while(true)
{
if(q->val < result->val)
result=result->left;
else if(p->val >result->val)
result=result->right;
else
return result;
}
}
};
LeetCode235:Lowest Common Ancestor of a Binary Search Tree