二叉树的先中后序遍历（非递归版）

const binaryTree = {
    val: 1,
    left: {
        val: 2,
        left: {
            val: 4,
        },
        right: {
            val: 5,
        }
    },
    right: {
        val: 3,
        left: {
            val: 6,
            left: {
                val: 8
            }
        },
        right: {
            val: 7,
        }
    }
}

1. 先序遍历

const preorder = (root) => {
    if(!root) return
    const stack = [root]
    while(stack.length) {
        const n = stack.pop()
        console.log(n.val)
        if (n.right) stack.push(n.right)
        if (n.left) stack.push(n.left)
    }
}

preorder(binaryTree)

2. 中序遍历

const inorder = (root) => {
    if (!root) return
    const stack = []
    let p = root
    while(stack.length || p) {
        while(p) {
            stack.push(p)
            p = p.left
        }
        const n = stack.pop()
        console.log(n.val)
        p = n.right
    }
}

inorder(binaryTree)

3. 后序遍历

const postorder = (root) => {
    if (!root) return
    const stack = [root]
    const res = []
    while(stack.length) {
        const n = stack.pop()
        res.push(n)
        if (n.left) stack.push(n.left)
        if (n.right) stack.push(n.right)
    }
    while(res.length) {
        const n = res.pop()
        console.log(n.val)
    }
}

postorder(binaryTree)