树中两个节点的最低公共祖先

树是二叉查找树的情况

题目来自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.
我们能够知道：
假设当前的节点的值比要查找的两个节点大的话，说明最低公共节点在当前节点的左子树。
假设当前的节点的值比要查找的两个节点小的话，说明最低公共节点在当前节点的右子树。

/**
 * 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;
    }
};

树是普通二叉树

题目来自于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;//假设当前节点为NULL说明走到了叶节点都没有找到两个节点中的当中一个
    if (root == p || root == q) 
        return root;//假设当前节点为p,q之一，那么返回当前找到的节点中的一个
    TreeNode *L = lowestCommonAncestor(root->left, p, q);//左子树中能否最先找到p,q中的一个节点
    TreeNode *R = lowestCommonAncestor(root->right, p, q);
    if (L && R) 
        return root;  //假设当前节点左右节点都各找到一个。那么返回当前节点
    return L ? L : R; //仅仅在左节点或者右节点找到一个。说明另一个节点是在当前节点的以下
    }
};

来源：https://www.cnblogs.com/slgkaifa/p/7297845.html