Leetcode 236 Solution

This article provides solution to leetcode question 236 (lowest-common-ancestor-of-a-binary-tree)
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https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree

Solution

/**
 * 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 {
    TreeNode* m_res;
public:
    int findLowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if (root == 0)
            return 0;

        int root_cnt = 0;

        if (root == p || root == q)
            root_cnt = 1;

        int left_cnt = findLowestCommonAncestor(root->left, p, q);
        int right_cnt = findLowestCommonAncestor(root->right, p, q);

        if ((left_cnt == 1 && right_cnt == 1) || (root_cnt == 1 && left_cnt + right_cnt == 1))
            m_res = root;

        return root_cnt + left_cnt + right_cnt;
    }

    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        findLowestCommonAncestor(root, p, q);

        return m_res;
    }
};