236 - Lowest Common Ancestor Of A Binary Tree & 235 - Lowest Common Ancestor Of A Binary Search Tree

236 - Lowest Common Ancestor of a Binary Tree

#medium

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).”

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.

使用後序遍歷(左右中)，需使用左右之結果來判斷(左右子樹是否為空)

/**
 * 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 == NULL ) return NULL;
        if ( root == p || root == q ) return root;
        TreeNode * left = lowestCommonAncestor( root -> left, p, q );
        TreeNode * right = lowestCommonAncestor( root -> right, p, q );
        if ( left != NULL && right != NULL ) return root;
        if ( left == NULL && right != NULL ) return right;
        else if ( left != NULL && right == NULL ) return left;
        else return NULL;
    }
};

235 - Lowest Common Ancestor of a Binary Search Tree

#medium

Given a binary search tree (BST), find the lowest common ancestor (LCA) node of two given nodes in the BST.

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).”

Example 1:

Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 8
Output: 6
Explanation: The LCA of nodes 2 and 8 is 6.

Example 2:

Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 4
Output: 2
Explanation: The LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

Solution 1

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;
    }
};

Solution 2

class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if ( root == NULL ) return NULL;
        if ( root == p || root == q ) return root;
        TreeNode * p_cur = NULL;
        TreeNode * q_cur = NULL;
        p_cur = lowestCommonAncestor( root -> left, p, q );
        q_cur = lowestCommonAncestor( root -> right, p, q );
        if ( p_cur != NULL && q_cur != NULL ) return root;
        if ( p_cur == NULL && q_cur != NULL ) return q_cur;
        else if ( p_cur != NULL && q_cur == NULL ) return p_cur;
        else return NULL;

    }
};