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116. Populating Next Right Pointers in Each Node

Posted on Edited on In Disqus:
Symbols count in article: 2.3k Reading time ≈ 2 mins.

O(n) time O(1) space

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/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* left;
    Node* right;
    Node* next;

    Node() : val(0), left(NULL), right(NULL), next(NULL) {}

    Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}

    Node(int _val, Node* _left, Node* _right, Node* _next)
        : val(_val), left(_left), right(_right), next(_next) {}
};
*/

class Solution {
public:
    Node* connect(Node* root) {
        Node dummy, *p = &dummy, *res = root;
        while (root) {
            if (root->left) {
                p->next = root->left;
                root->left->next = root->right;
                p = root->right;
            }
            root = root->next;
            if (!root) {
                root = dummy.next;
                dummy.next = nullptr;
                p = &dummy;
            }
        }
        return res;
    }
};

直接用117. Populating Next Right Pointers in Each Node II的方法也行

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/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* left;
    Node* right;
    Node* next;

    Node() : val(0), left(NULL), right(NULL), next(NULL) {}

    Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}

    Node(int _val, Node* _left, Node* _right, Node* _next)
        : val(_val), left(_left), right(_right), next(_next) {}
};
*/

class Solution {
public:
    Node* connect(Node* root) {
        Node dummy, *p = &dummy, *res = root;
        while (root) {
            if (root->left) {
                p->next = root->left;
                p = p->next;
            }
            if (root->right) {
                p->next = root->right;
                p = p->next;
            }
            root = root->next;
            if (!root) {
                root = dummy.next;
                dummy.next = nullptr;
                p = &dummy;
            }
        }
        return res;
    }
};

O(n) time O(h) space

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/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* left;
    Node* right;
    Node* next;

    Node() {}

    Node(int _val, Node* _left, Node* _right, Node* _next) {
        val = _val;
        left = _left;
        right = _right;
        next = _next;
    }
};
*/
class Solution {
public:
    Node* connect(Node* root) {
        if (!root) return root;
        Node dummy_head, *tail = &dummy_head;
        for (auto p = root; p; p = p->next) {
            if (p->left) {
                tail->next = p->left;
                tail = tail->next;
            }
            if (tail->next) break;
            if (p->right) {
                tail->next = p->right;
                tail = tail->next;
            }
            if (tail->next) break;
        }
        connect(root->right);
        connect(root->left);
        return root;
    }
};