c_cpp LCA，lca，二叉树的最低共同的ancester

Posted 2021-05-14

// ====================================
// LCA of a nornal Binary Tree, no parent pointers
// solution 1. But easy to TIMEOUT!
bool contain(TreeNode *root, TreeNode *p) {
    if(!root) return false;
    if(root == p) return true;
    return contain(root->left, p) || contain(root->right, p);
}

TreeNode* LCA(TreeNode *root, TreeNode *p, TreeNode *q) {
    if(!root) return NULL;
    if(contain(root->left, p) && contain(root->left, q))
        return LCA(root->left, p, q);
    if(contain(root->right, p) && contain(root->right, q))
        return LCA(root->right, p, q);
    return root;
}

// ====================================
// solution 2, recursive, time O(n)
TreeNode* LCA2(TreeNode *root, Node *p, Node *q) {
    if(!root) return NULL;
    if(root == p || root == q) return root;
    TreeNode *L = LCA2(root->left, p, q);
    TreeNode *R = LCA2(root->right, p, q);
    if(L&&R) return root;
    if(L && !R) return L;
    if(!L && R) return R;
    return NULL;
}

// ====================================
// LCA of a nornal Binary Tree, has parent pointer, 
// solution 1: calculate each depth, then walk up. O(1) space, O(max(height(p), height(q))) time.
struct TreeNode {
    int val;
    TreeNode *left, *right, *parent;
};

TreeNode* LCA(TreeNode *root, TreeNode *p, TreeNode *q) {
    int depth1 = 0, depth2 = 0;
    TreeNode *pp = p, *qq = q;
    while(pp) {
        pp = pp->parent;
        ++depth1;
    }
    while(qq) {
        qq = qq->parent;
        ++depth2;
    }
    TreeNode *u = depth1 > depth2 ? p : q;
    TreeNode *v = depth1 > depth2 ? q : p;
    int diff = abs(depth1 - depth2);
    while(diff--) t = t->parent;
    while(u != v) {
        u = u->parent;
        v = v->parent;
    }
    return u;
}

// ====================================
// LCA of a nornal Binary Tree, has parent pointer, 
// solution 2: use hash table. O(max(dp-dl, dq-dl)) space, O(max(dp-dl, dq-dl)) time.
// time is better than the above solution 1. trade space for time.
TreeNode* LCA(TreeNode *root, TreeNode *p, TreeNode *q) {
    unordered_set<TreeNode*> hash;
    ...
}



// ====================================
// LCA of a Binary Search Tree (BST), no parent pointers
TreeNode* LCA3(TreeNode *root, TreeNode *p, TreeNode *q) {
    if(!root) return NULL;
    if(!p) return q;
    if(!q) return p;
    int vp = p->val, vq = q->val, vr = root->val;
    if(vr < vp && vr < vq) 
        return LCA3(root->right, p, q);
    if(vr > vp && vr > vq) 
        return LCA3(root->left, p, q);
    return root;
}