题目描述
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 p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”
Given the following binary tree: root = [3,5,1,6,2,0,8,null,null,7,4]
Example 1:
Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
Output: 3
Explanation: The LCA of nodes 5 and 1 is 3.
Example 2:
Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
Output: 5
Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.
Note:
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All of the nodes' values will be unique.
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p and q are different and both values will exist in the binary tree.
难度系数 Medium
解法一:简单的dfs
class Solution { public: TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) { if (!root || root==p || root==q) return root; //得到的是子节点 TreeNode* left = lowestCommonAncestor(root->left, p, q); TreeNode* right = lowestCommonAncestor(root->right, p, q); //返回父节点 if (left && right) return root; return left ? left : right; } };
解法二:对上个算法的优化
class Solution { public: TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) { if (!root || p == root || q == root) return root; TreeNode *left = lowestCommonAncestor(root->left, p, q); //表示已找到节点,不需要再找 if (left && left != p && left != q) return left; TreeNode *right = lowestCommonAncestor(root->right, p , q); if (left && right) return root; return left ? left : right; } };
github地址:https://github.com/AntonioSu/leetcode/blob/master/problems/236.LowestCommonAncestorofaBinaryTree.md