樹是二叉查詢樹的情況

題目來自LeetCode：https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/
Lowest Common Ancestor of a Binary Search Tree Total Accepted: 3402 Total Submissions: 8709 My Submissions Question Solution
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.
我們可以知道：
如果當前的節點的值比要查詢的兩個節點大的話，說明最低公共節點在當前節點的左子樹。
如果當前的節點的值比要查詢的兩個節點小的話，說明最低公共節點在當前節點的右子樹。

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

樹是普通二叉樹

題目來自於LeetCode：https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/
Lowest Common Ancestor of a Binary Tree Total Accepted: 700 Total Submissions: 2270 My Submissions Question Solution
Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

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).”

    _______3______
   /              \
___5__          ___1__

/ \ / \
6 _2 0 8
/ \
7 4
For example, the lowest common ancestor (LCA) of nodes 5 and 1 is 3. Another example is LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

 /**
 * 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) 
        return NULL;//如果當前節點為NULL說明走到了葉節點都沒有找到兩個節點中的其中一個
    if (root == p || root == q) 
        return root;//如果當前節點為p,q之中的一個，那麼返回當前找到的節點中的一個
    TreeNode *L = lowestCommonAncestor(root->left, p, q);//左子樹中是否能最先找到p,q中的一個節點
    TreeNode *R = lowestCommonAncestor(root->right, p, q);
    if (L && R) 
        return root;  //如果當前節點左右節點都各找到一個，那麼返回當前節點
    return L ? L : R; //只在左節點或者右節點找到一個，說明還有一個節點是在當前節點的下面
    }
};