剑指offer系列-68 I 二叉搜索树的最近公共祖先

// 解法一：递归
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if(root==nullptr)
            return nullptr;
        if(p->val < root->val && q->val < root->val)
            return lowestCommonAncestor(root->left, p, q);
        else if(p->val > root->val && q->val > root->val)
            return lowestCommonAncestor(root->right, p, q);
        return root;
    }
};


// 解法二：类似二分法查找
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        TreeNode* ans=root;
        while(true){
            if(p->val<ans->val && q->val<ans->val){
                ans=ans->left;
            }
            else if(p->val>ans->val && q->val>ans->val){
                ans=ans->right;
            }
            else{
                break;
            }
        }
        return ans;
    }
};


// 解法三：未利用二叉搜索树的性质
class Solution {
public:
    TreeNode* ans;
    bool dfs(TreeNode* root, TreeNode* p, TreeNode* q){
        if(root==nullptr)  return false;
        int lson=dfs(root->left,p,q);
        int rson=dfs(root->right,p,q);
        if((lson&&rson) || ((root->val==p->val || root->val==q->val) && (lson || rson))){
            ans=root;
        }
        return lson || rson || (root->val==p->val || root->val==q->val);
    }

    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        dfs(root, p, q);
        return ans;
    }
};