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 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;  
  *   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 NULL;  
     }else{  
       return left != NULL ? left : right;  
     }  
   }  
 };