Lintcode 3715 · Lowest Common Ancestor V (最小祖先好题)

3715 · Lowest Common Ancestor VPRE

Algorithms

Medium

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Description

Given a binary tree with a root node root and an array nodes of objects of class TreeNode, you need to find the Lowest Common Ancestor (LCA, Lowest Common Ancestor) of all nodes in nodes and return it.

Where all the nodes in nodes exist in that binary tree and all the nodes in the binary tree have different values from each other.

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The number of nodes in the tree is in the range

1 , 1 0 4

1,10 4

−

≤

�

≤

−10

≤TreeNode.val≤10

All TreeNode.val are unique

All nodes[i] are unique

All nodes[i] exist in the tree

Example

Example 1

Input

root = {3,5,1,6,2,0,8,#,#,7,4}

nodes = [4,7]

Output

Explanation

The lowest common ancestor of TreeNode(4) and TreeNode(7) is TreeNode(2)

Example 2

Input

root = {3,5,1,6,2,0,8,#,#,7,4}

nodes = [2]

Output

Explanation

The lowest common ancestor of a single node is the node itself

Example 3

Input

root = {3,5,1,6,2,0,8,#,#,7,4}

nodes = [7,2,6,4]

Output

Explanation

The lowest common ancestor of TreeNode(7)、TreeNode(2)、TreeNode(6) and TreeNode(4) is TreeNode(5)

Example 4

Input

root = {3,5,1,6,2,0,8,#,#,7,4}

nodes = [0,2,3,1,5,8,6,4,7]

Output

Explanation

nodes contains all the nodes in the tree, and the lowest common ancestor of all the nodes in the tree is the root node

Related Problems

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Lowest Common Ancestor II

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578

Lowest Common Ancestor III

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3715

Lowest Common Ancestor V

Medium

解法1：套用的labuladong的模板。

cpp 复制代码

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @param root: The root node of a binary tree.
     * @param nodes: An array of objects of class TreeNode.
     * @return: The lowest common ancestor of nodes.
     */
    TreeNode* lowestCommonAncestor(TreeNode *root, vector<TreeNode*> &nodes) {
        if (!root) return NULL;
        unordered_set<TreeNode *> nodeSet;
        for (auto pNode : nodes) nodeSet.insert(pNode);
        TreeNode *res = helper(root, nodeSet);
        return res;
    }
private:
    TreeNode* helper(TreeNode *root, unordered_set<TreeNode *> &nodeSet) {
        if (!root) return NULL;
        if (nodeSet.find(root) != nodeSet.end()) return root;
        TreeNode *left = NULL, *right = NULL;
        
        left = helper(root->left, nodeSet);
        right = helper(root->right, nodeSet);
        
        if (left && right) return root;
        return left ? left : right;
    }
};