Lowest Common Ancestor of a Binary Search Tree & a Binary Tree
235. Lowest Common Ancestor of a Binary Search Tree
題目鏈接:https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/#/description
題目大意:給定一棵二叉查找樹和兩個節點p和q,要求返回這兩個節點的第一個公共祖先。
思路:對於當前節點node,如果p和q的val都大於node的val,則說明p和q的第一個公共祖先在node的右子樹,如果p和q的val都小於node的val,則說明p和q的第一個公共祖先在node的左子樹,否則node就是p和q的第一個公共祖先。
算法步驟:(1)如果root->val > max(p->val, q->val),令root = root->left,調回步驟(1),否則,如果root->val < min(p->val, q->val),令root = root->right,調回步驟(1),否則返回root。
算法復雜度:時間復雜度為O(log(n)),空間復雜度為O(1)
代碼:
1 /** 2 * Definition for a binary tree node. 3 * struct TreeNode { 4 * int val;5 * TreeNode *left; 6 * TreeNode *right; 7 * TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8 * }; 9 */ 10 class Solution { 11 public: 12 TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) { 13 while (true) { 14 if (root->val > max(p->val, q->val))15 root = root->left; 16 else if (root->val < min(p->val, q->val)) 17 root = root->right; 18 else 19 return root; 20 } 21 } 22 };
評測系統上運行結果:
236. Lowest Common Ancestor of a Binary Tree
題目鏈接:https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/#/description
題目大意:給定一棵二叉樹和兩個節點p和q,要求返回這兩個節點的第一個公共祖先。
思路:如果p和q都在某節點的左邊,就到左子樹中查找共同祖先。若都在右邊,則在右子樹中查找共同祖先。如果p和q不在同一邊,那就表示已經找到第一個共同祖先。
復雜度分析:時間復雜度為O(n),空間復雜度為O(n)
代碼:
1 /** 2 * Definition for a binary tree node. 3 * struct TreeNode { 4 * int val; 5 * TreeNode *left; 6 * TreeNode *right; 7 * TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8 * }; 9 */ 10 class Solution { 11 public: 12 TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) { 13 if (!root || root == p || root == q) 14 return root; 15 auto left = lowestCommonAncestor(root->left, p, q); 16 auto right = lowestCommonAncestor(root->right, p, q); 17 if (left && right) 18 return root; 19 return left ? left : right; 20 } 21 };
評測系統上運行結果:
Lowest Common Ancestor of a Binary Search Tree & a Binary Tree