Lowest Common Ancestor of a Binary Tree

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 v and w as the lowest node in T that has both v and w as descendants (where we allowa node to be a descendant of itself).”

        _______3______
       /              \
    ___5__          ___1__
   /      \        /      \
   6      _2       0       8
         /  \
         7   4

For example, the lowest common ancestor (LCA) of nodes 5 and 1 is 3. Another example is LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

/**
 * 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) {
        TreeNode *result = NULL;  
        findLCA(root, p, q, &result);  
  
        return result;  
    }
private:  
    bool findNode(TreeNode *root, TreeNode *p)  
    {  
        if (root == NULL || p == NULL)  
        {  
            return false;  
        }  
  
        if (root == p)  
        {  
            return true;  
        }  
  
        return findNode(root->left, p) || findNode(root->right, p);  
    }  
  
    int findLCA(TreeNode *root, TreeNode *p, TreeNode *q, TreeNode **result)  
    {  
        if (root == NULL)  
        {  
            return 0;  
        }  
  
        if (root == p)  
        {  
            if (findNode(root, q))  
            {  
                *result = root;  
                return 2;  
            }  
            else  
            {  
                return 1;  
            }  
        }  
        else if (root == q)  
        {  
            if (findNode(root, p))  
            {  
                *result = root;  
                return 2;  
            }  
            else  
            {  
                return 1;  
            }  
        }  
        else   
        {  
            int left = findLCA(root->left, p, q, result);  
            int right = 0;  
            if (left != 2)  
            {  
                right = findLCA(root->right, p, q, result);  
            }  
            if (left == 1 && right == 1)  
            {  
                *result = root;  
            }  
            return left + right;  
        }  
    }  
};