数据结构 - 把二叉查找树转变成排序的双向链表（C++）

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/*
 * 把二元查找树转变成排序的双向链表 - by Chimomo
 *
 * 【思路】按照二叉树的中序遍历正好是按从小到大地顺序遍历全部节点。在遍历的过程中，更改它的逻辑结构。
 */

#include <iostream>

using namespace std;

/**
 * The binary search tree node.
 */
struct BSTreeNode {
    int m_nValue; // The value of node.
    BSTreeNode *m_pLeft; // The left child of node.
    BSTreeNode *m_pRight; // The right child of node.
};

typedef BSTreeNode DoublyLinkedList;
DoublyLinkedList *pHead = nullptr;
DoublyLinkedList *pDoublyLinkedList = nullptr;

/**
 * Construct binary search tree recursively.
 * @param pCurrent The current pointer. Use & here since pCurrent will be changed in the function body.
 * @param value The value.
 */
void ConstructBSTree(BSTreeNode *&pCurrent, int value) {
    if (pCurrent == nullptr) {
        auto pTree = new BSTreeNode();
        pTree->m_pLeft = nullptr;
        pTree->m_pRight = nullptr;
        pTree->m_nValue = value;
        pCurrent = pTree;
    } else {
        if (value > pCurrent->m_nValue) {
            ConstructBSTree(pCurrent->m_pRight, value);
        } else if (value < pCurrent->m_nValue) {
            ConstructBSTree(pCurrent->m_pLeft, value);
        } else {
            cout << "Invalid input, there already have the value!" << endl;
        }
    }
}

/**
 * Convert binary search tree node to doubly linked list node.
 * @param pCurrent The current pointer.
 */
void ConvertBSTreeNodeToDoublyLinkedListNode(BSTreeNode *pCurrent) {
    pCurrent->m_pLeft = pDoublyLinkedList;

    if (pDoublyLinkedList == nullptr) {
        pHead = pCurrent;
    } else {
        pDoublyLinkedList->m_pRight = pCurrent;
    }

    pDoublyLinkedList = pCurrent;
}

/**
 * Traverse binary search tree (in-order) to convert binary tree to doubly linked list.
 * @param pCurrent The current pointer.
 */
void TraverseBSTreeInOrder(BSTreeNode *pCurrent) {
    if (pCurrent == nullptr) {
        return;
    } else {
        if (pCurrent->m_pLeft != nullptr) {
            TraverseBSTreeInOrder(pCurrent->m_pLeft);
        }

        ConvertBSTreeNodeToDoublyLinkedListNode(pCurrent);

        if (pCurrent->m_pRight != nullptr) {
            TraverseBSTreeInOrder(pCurrent->m_pRight);
        }
    }
}

/**
 * Test program.
 * @param argc The argument count.
 * @param argv The argument array.
 * @return The int.
 */
int main(int argc, char *argv[]) {
    int value;
    BSTreeNode *pBSTree = nullptr;
    cout << "Please input integers, -1 to end:";
    cin >> value;

    while (-1 != value) {
        ConstructBSTree(pBSTree, value);
        cin >> value;
    }

    TraverseBSTreeInOrder(pBSTree);

    cout << "Print BST:";
    while (pHead) {
        cout << pHead->m_nValue << ' ';
        pHead = pHead->m_pRight;
    }

    return 0;
}

// Output:
/*
Please input integers, -1 to end:6 7 8 5 1 3 9 2 4 0 10 -1
6 7 8 5 1 3 9 2 4 0 10 -1
Print BST:0 1 2 3 4 5 6 7 8 9 10
*/