二叉树，先中后遍历非递归版

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

//先序遍历
const preorder = (root) => {
    if (!root) { return }
    const stack = [root];
    while (stack.length > 0) {
        const n = stack.pop()
        console.log(n.val)
        if (n.right) stack.push(n.right)
        if (n.left) stack.push(n.left)
    }
}

//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
    }
}

//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)
    }
}