二叉树祖先问题

//找到第一个共同祖先

class Solution{
	//若p为root的子孙，则返回true
	bool covers(TreeNode*root, TreeNode*p){
		if(NULL == root)
			return false;
		if(root == p)
			return true;
		
		return covers(root->left,p)||covers(root->right,p);
	}
	
	TreeNode* commonAncestorHelper(TreeNode*root, TreeNode*p, TreeNode*q){
		if(root==NULL)
			return NULL;
		if(root==p || root==q)
			return root;
		
		bool is_p_on_left=covers(root->left,p);
		bool is_q_on_right=covers(root->right,q);
		
		//若p和q不在同一边，则返回root
		if(is_p_on_left != is_q_on_right)
			return root;
		
		//否则就在同一边，遍访那一边
		TreeNode* child_side = is_p_on_left?root->left:root->right;
		return commonAncestorHelper(child_side,p,q);
	}
	
	TreeNode* commonAncetor(TreeNode*root, TreeNode*p, TreeNode*q){
		if(!covers(root,p) || !covers(root,q))	//错误检查
			return NULL;
		
		return commonAncestorHelper(root,p,q);
	}
};

//找到最近祖先
class Solution{
	TreeNode* lowestCommonAnceston(TreeNode* root, TreeNode* A, TreeNode* B){
		if(NULL==root || root==A || root==B)
			return NULL;
		
		TreeNode* left = lowestCommonAnceston(root->left,A,B);
		TreeNode* root = lowestCommonAnceston(root->right,A,B);
		if(left && right)
			return root;
		
		return left?left:right;
		
	}
	
};