leetcode 235. The nearest common ancestor of a binary search tree

Given a binary search tree, find the nearest common ancestor of two specified nodes in the tree.

The definition of the nearest common ancestor in Baidu Encyclopedia : "For two nodes u and v of a rooted tree T, the nearest common ancestor represents a node x, which satisfies that x is the ancestor of u and v and the depth of x is as large as possible." (A node can also be its own ancestor)

        _______6______
       /              \
    ___2__          ___8__
   /      \        /      \
   0      _4       7       9
         /  \
         3   5

For example, the node  2 and  8the nearest common ancestor are  6. As another example, the last common ancestor of a node  2 sum  4 is  2, because by definition the last common ancestor of a node can be the specified node itself.

 

Ideas:

• If the value of root is equal to the value of p, q, then return root

• If the value of root is in the middle of the value of p, q, then the root is the common ancestor of p, q

• If the value of root is greater than the value of p and q, find the common ancestor of p, q in the left subtree of root

• If the value of root is greater than the values ​​of p and q, find the common ancestor of p, q in the right subtree of root 

  　

1 class Solution {
2 public:
3     TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
4         if(root->val == p->val || root->val == q->val) return root;
5         if((root->val > p->val && root->val < q->val)||(root->val < p->val && root->val > q->val)) return root;
6         if(root->val < p->val && root->val < q->val) return lowestCommonAncestor(root->right, p, q);
7         if(root->val > p->val && root->val > q->val) return lowestCommonAncestor(root->left, p,q);
8     }
9 };