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1644. Lowest Common Ancestor of a Binary Tree II

Posted on Edited on In Disqus:
Symbols count in article: 1.4k Reading time ≈ 1 mins.

236. Lowest Common Ancestor of a Binary Tree的followup
postorder O(n) time O(h) space

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        auto res = lca(root, p, q);
        return cnt == 2 ? res : nullptr;
    }

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

    int cnt = 0;
};
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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        auto res = lca(root, p, q);
        return cnt == 2 ? res : nullptr;
    }

    TreeNode *lca(TreeNode* root, TreeNode* p, TreeNode* q) {
        if (!root) return root;
        if (root == p || root == q) {
            if (++cnt == 2) return root;
        }
        auto l = lca(root->left, p, q);
        auto r = lca(root->right, p, q);
        if (cnt == 1) return root;
        if (l && r) return root;
        if (cnt == 2) return (root == p || root == q) ? root : (l ? l : r);
        return nullptr;
    }

    int cnt = 0;
};