树中两个节点的最低公共祖先
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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.
我们可以知道:
如果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。
如果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;//如果当前节点为NULL说明走到了叶节点都没有找到两个节点中的其中一个 if (root == p || root == q) return root;//如果当前节点为p,q之中的一个,那么返回当前找到的节点中的一个 TreeNode *L = lowestCommonAncestor(root->left, p, q);//左子树中是否能最先找到p,q中的一个节点 TreeNode *R = lowestCommonAncestor(root->right, p, q); if (L && R) return root; //如果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; //只在左节点或者右节点找到一个,说明还有一个节点是在当前节点的下面 }};
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