LeetCode 236. Lowest Common Ancestor of a Binary Tree; 235. Lowest Common Ancestor of a Binary Search Tree

236. Lowest Common Ancestor of a Binary Tree

递归寻找p或q，如果找到，层层向上返回，知道 root 左边和右边都不为NULL：if (left!=NULL && right!=NULL) return root;

时间复杂度 O(n)，空间复杂度 O(H)

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) return left;
        if (right!=NULL) return right;
        return NULL;
    }
};

 

235. Lowest Common Ancestor of a Binary Search Tree

实际上比上面那题简单，因为LCA的大小一定是在 p 和 q 中间，只要不断缩小直到在两者之间就行了。

class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if (root->val>p->val && root->val>q->val)
            return lowestCommonAncestor(root->left,p,q);
        if (root->val<p->val && root->val<q->val)
            return lowestCommonAncestor(root->right,p,q);
        return root;
    }
};