Lowest Common Ancestor of a Binary Tree leetcode

Posted 2020-06-11

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

        _______3______
       /                  ___5__          ___1__
   /      \        /         6      _2       0       8
         /           7   4

For example, the lowest common ancestor (LCA) of nodes 5 and 1 is 3. Another example is LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

 

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非递归实现：

TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
    if (root == nullptr)
        return root;
    stack<TreeNode*> sta;
    vector<TreeNode*> vec;
    bool tag1 = false;
    bool tag2 = false;
    sta.push(root);
    TreeNode* lastRoot = root;
    while (!sta.empty())
    {
        root = sta.top();    
        if (root == p) {
            if(tag1 == false && tag2 == false)
                vec.push_back(root);
            tag1 = true;
        }
        else if (root == q) {
            if (tag2 == false && tag1 == false)
                vec.push_back(root);
            tag2 = true;
        }
        if (!tag1 && !tag2)
            vec.push_back(root);
        if (tag1 && tag2 && find(vec.begin(), vec.end(), root) != vec.end())
            return root;
                
        if (lastRoot != root->right)
        {
            if (lastRoot != root->left) {
                if (root->left != nullptr) {                    
                    sta.push(root->left);
                    continue;
                }
            }
            if (root->right != nullptr) {
                sta.push(root->right);
                continue;
            }
        }
        lastRoot = root;
        sta.pop();
    }
    return nullptr;
}

递归实现：

TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
    if (!root || root == p || root == q) return root;
    TreeNode* l = lowestCommonAncestor(root->left, p, q);
    TreeNode* r = lowestCommonAncestor(root->right, p, q);
    return l && r ? root : l ? l : r;
}