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 allow a 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;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
struct TreeNode* lowestCommonAncestor(struct TreeNode* root, struct TreeNode* p, struct TreeNode* q) {
    struct TreeNode *left, *right;
    if(NULL == root){
            return NULL;
        }
        if(root == p||root == q){
            return root;
        }
        left = lowestCommonAncestor(root->left, p, q);
        right = lowestCommonAncestor(root->right, p, q);
        if(NULL == left){
            return right;
        }
        if(NULL == right){
            return left;
        }
        return root;
}

LeetCode OJ 236. Lowest Common Ancestor of a Binary Tree