二叉树遍历算法

1. 前序/中序/后序遍历（递归实现）

// 前序遍历
void BT_PreOrder(BiTreePtr pNode)
{
    if (!pNode)  return;
     
    visit(pNode);
    BT_PreOrder(pNode->left);
    BT_PreOrder(pNode->right);   
}
// 中序遍历
void BT_PreOrder(BiTreePtr pNode)
{
    if (!pNode)  return;
     
    BT_PreOrder(pNode->left);
    visit(pNode);
    BT_PreOrder(pNode->right);
}
// 后序遍历
void BT_PreOrder(BiTreePtr pNode)
{
    if (!pNode)  return;
     
    BT_PreOrder(pNode->left);
    BT_PreOrder(pNode->right);
    visit(pNode);
}

2. 前序遍历（非递归实现）

// 用栈实现
void BT_PreOrderNoRec1(BiTreePtr pNode)
{
    stack<BiTreePtr> s;
    while (!pNode || !s.empty())
    {
        if (!pNode)
        {
            visit(pNode);
            s.push(pNode);
            pNode = pNode->left;
        }
        else
        {
            pNode = s.pop();
            pNode = pNode->right;
        }
    }
}
 
// 用栈实现
void BT_PreOrderNoRec2(BiTreePtr pNode)
{
    if (!pNode)
    {
        stack<BiTreePtr> s;
        s.push(pNode);
        while (!s.empty())
        {
            BiTreePtr pvNode = s.pop(); 
            visit(pvNode);
            s.push(pvNode->right);
            s.push(pvNode->left);
        }
    }
}
 
// 不用栈实现 每个节点含父节点指针和isVisited【默认为false】状态变量 且该二叉树含一个头节点
void BT_PreOrderNoRec3(BiTreePtr pNode)
{
    while (!pNode)// 回溯到指向根节点的头节点时退出
    {
        if( !pNode->bVisited )// 判定是否已被访问
        {   
            visit(pNode);
            pNode->isVisited = true;
        }
        if ( pNode->left && !pNode->left->isVisited )
            pNode = pNode->left;
        else if( pNode->right && !pNode->right->isVisited )
            pNode = pNode->right;
        else   //回溯
            pNode = pNode->parent;
    }
}

3. 中序遍历（非递归实现）

// 用栈实现
void BT_InOrderNoRec1(BiTreePtr pNode)
{
    stack<BiTreePtr> s;
    while (!pNode || !s.empty())
    {
        if (!pNode)
        {
            s.push(pNode);
            pNode = pNode->left;
        }
        else
        {
            pNode = s.pop();
            visit(pNode);
            pNode = pNode->right;
        }
    }
}
// 不用栈实现 每个节点含父节点指针和isVisited【默认为false】的状态变量 且该二叉树含一个头节点
void BT_InOrderNoRec2(BiTreePtr pNode)
{
    while (!pNode) // 回溯到指向根节点的头节点时退出
    {
        while (pNode->left && !pNode->left->isVisited)
            pNode = pNode->left;
        if (!pNode->isVisited)
        {
            visit(pNode);
            pNode->isVisited=true;
        }
        if (pNode->right && !pNode->right->isVisited) 
            pNode = pNode->right;
        else
            pNode = pNode->parent;
    }
}

4. 后序遍历（非递归实现）

void BT_PostOrderNoRec(BiTreePtr pNode)
{
    if(!pNode) return;
 
    stack<BiTreePtr> s;
    s.push(pNode);
 
    while (!s.empty())
    {
        BiTreePtr pvNode = s.pop(); 
        if (pvNode->isPushed)// 表示其左右子树都已入栈，访问该节点
            visit(pvNode);
        else
        {
            if (pvNode->right)
            {
                pvNode->right->isPushed = false;
                S.push(pvNode->right);
            }
            if (pvNode->left)
            {
                pvNode->left->isPushed = false;
                s.push(pvNode->left);
            }
            pvNode->isPushed = true;
            s.push(pvNode);
        }
    }
}

5. 层序遍历（使用队列）

void BT_LevelOrder(BiTreePtr pNode)
{
    if (!pNode) return;
 
    queue<BiTreePtr> q; 
    q.push(pNode);
 
    BiTreePtr pvNode;
    while (!q.empty())
    {
        pvNode = q.pop();
        visit(pvNode);
 
        if (pvNode->left)
            q.push(pvNode->left);    
        if (pvNode->right)
            q.push(pvNode->right);            
    }
}