力扣103--二叉树的锯齿形遍历

题目描述

给定一个二叉树，返回其节点值的锯齿形层次遍历。（即先从左往右，再从右往左进行下一层遍历，以此类推，层与层之间交替进行）。

分析思路

1.双端队列

def zigzagLevelOrder(self, root: TreeNode) -> List[List[int]]:
        if not root:
            return []
        q=collections.deque()
        q.append(root)
        mark=0
        res=[]
        while q:
            level=[]
            if mark%2==0:
                for _ in range(len(q)):
                    node=q.popleft()
                    level.append(node.val)
                    if node.left: q.append(node.left)
                    if node.right: q.append(node.right)
                res.append(level)
            else:
                for _ in range(len(q)):
                    node=q.pop()
                    level.append(node.val)
                    if node.right: q.appendleft(node.right)
                    if node.left: q.appendleft(node.left)
                res.append(level)
            mark+=1
        return res

2.两个栈实现

python:

def Print( pRoot):
    # write code here
    res = []
    if pRoot == None:
        return res
    stack1 = []
    stack2 = []
    stack1.append(pRoot)
    level = []
    while stack1 or stack2:
        while stack1:
            cur = stack1[-1]
            if cur.left != None:
                stack2.append(cur.left)
            if cur.right != None:
                stack2.append(cur.right)
            level.append(cur.val)
            stack1.pop()
        if level:
            res.append(level)
            level = []
        while stack2:
            cur = stack2[-1]
            if cur.right != None:
                stack1.append(cur.right)
            if cur.left != None:
                stack1.append(cur.left)
            level.append(cur.val)
            stack2.pop()
        if level:
            res.append(level)
            level = []
    return res

c++

vector > Print(TreeNode* pRoot) {
        vector> res;
        if (pRoot==NULL)
            return res;
        stack stack1,stack2;
        stack1.push(pRoot);
        vector tmp;
        while(!stack1.empty() || !stack2.empty()){
            TreeNode* cur=nullptr;
            while(!stack1.empty()){
                cur=stack1.top();
                if(cur->left !=NULL)
                    stack2.push(cur->left);
                if(cur->right !=NULL)
                    stack2.push(cur->right);
                tmp.push_back(cur->val);
                stack1.pop();
            }
            if(tmp.size()){
                res.push_back(tmp);
                tmp.clear();
            }
            while(!stack2.empty()){
                cur=stack2.top();
                if(cur->right !=NULL)
                    stack1.push(cur->right);
                if(cur->left !=NULL)
                    stack1.push(cur->left);
                tmp.push_back(cur->val);
                stack2.pop();
            }
            if(tmp.size()){
                res.push_back(tmp);
                tmp.clear();
            }
        }
        return res;
    }

3.输出结果

完整例子如下

def Print(pRoot):
    # write code here
    res = []
    if pRoot == None:
        return res
    stack1 = []
    stack2 = []
    stack1.append(pRoot)
    while stack1 or stack2:
        while stack1:
            cur = stack1[-1]
            if cur.left != None:
                stack2.append(cur.left)
            if cur.right != None:
                stack2.append(cur.right)
            res.append(cur.val)
            stack1.pop()
        while stack2:
            cur = stack2[-1]
            if cur.right != None:
                stack1.append(cur.right)
            if cur.left != None:
                stack1.append(cur.left)
            res.append(cur.val)
            stack2.pop()
    return res
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None
def main():
    t1 = TreeNode(1)
    t2 = TreeNode(2)
    t3 = TreeNode(3)
    t4 = TreeNode(4)
    t5 = TreeNode(5)
    t6 = TreeNode(6)
    t7 = TreeNode(7)

    t1.left = t2
    t1.right = t3
    t2.left = t4
    t2.right = t5
    t3.left = t6
    t4.right = t7
    print(Print(t1))
if __name__ == "__main__":
    main()