236. Lowest Common Ancestor of a Binary Tree

遞歸尋找p或q，如果找到，層層向上返回，知道 root 左邊和右邊都不為NULL：if (left!=NULL && right!=NULL) return root;

時間復雜度 O(n)，空間復雜度 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

實際上比上面那題簡單，因為LCA的大小一定是在 p 和 q 中間，只要不斷縮小直到在兩者之間就行了。

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;
    }
};

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