利用栈实现二叉树的前序、中序、后序遍历

/**

* 注意：left/right值若没有显示设置为null，值即为undefined

* 在调用二叉树前、中、后序遍历方法时，由于参数设置了默认值（tree）

* 所以进入了死循环

*/

const tree = {
    value: 5,
    left: {
        value: 3,
        left: {
            value: 2,
            left: null,
            right: null
        },
        right: {
            value: 4,
            left: null,
            right: null
        }
    },
    right: {
        value: 7,
        left: {
            value: 6,
            left: null,
            right: null
        },
        right: {
            value: 8,
            left: null,
            right: null
        }
    }
}

/**

* 前序遍历

* root -> left -> right

*/

const preOrderTraverse = (node = tree, cb = () => {}) => {
    if(!node) return
    cb(node.value)
    preOrderTraverse(node.left)
    preOrderTraverse(node.right)
 

/**

* 中序遍历

* left -> root -> right

*/

const inOrderTraverse = (node = tree, cb = () => {}) => {
    if(!node) return
    inOrderTraverse(node.left)
    cb(node.value)
    inOrderTraverse(node.right)
}

/**

* 后续遍历

* left -> right -> root

*/

const postOrderTraverse = (node = tree, cb= () => {}) => {
    if(!node) return
    postOrderTraverse(node.left)
    postOrderTraverse(node.right)
    cb(node.value)
}

前序：A->B->D->E->C->F->G

function preTraverStack(root,cb){
    let stack = new Stack()
    stack.push(root)
    while(!stack.isEmpty()){
        let node = stack.pop()
        if(node.right != null){
            stack.push(node.right)
        }
        if(node.left != null){
            stack.push(node.left)
        }
        cb(node.val)
    }
}

function preTraver(root,cb){
    let arr = []
    arr.push(root)
    while(arr.length != 0){
        let node  = arr.pop()
        if(node.right != null){
            arr.push(node.right)
        }
        if(node.left != null){
            arr.push(node.left)
        }
        cb(node.val)
    }
}

中序：D->B->E->A->F->C->G

function inTraverStack(root,cb){
    let stack = new Stack()
    stack.push(root)
    while(!stack.isEmpty()){
        while(stack.peek().left != null){
            stack.push(stack.peek().left)
        }
        while(!stack.isEmpty()){
            let node = stack.pop()
            cb(node.val)
            if(node.right != null){
                stack.push(node.right)
                break
            }
        }
    }
}

function inTraver(root,cb){
    let arr = []
    arr.push(root)
    while(arr.length != 0){
        while(arr[arr.length - 1].left != null){
            arr.push(arr[arr.length - 1].left)
        }
        while(arr.length != 0){
            let node = arr.pop()
            cb(node.val)
            if(node.right != null){
                arr.push(node.right)
                break
            }
        }
    }
}

后序：D->E->B->F->G->C->A

function postTraverStack(root,cb){
    let stack = new Stack()
    stack.push(root)
    let lastNode = null
    while(!stack.isEmpty()){
        while(stack.peek().left != null){
            stack.push(stack.peek().left)
        }
        while(!stack.isEmpty()){
            if(lastNode == stack.peek().right || stack.peek().right == null){
                let node = stack.pop()
                cb(node.val)
                lastNode = node
            }else if(stack.peek().right != null){
                stack.push(stack.peek().right)
                break
            }
        }
    }
}