二叉树非递归遍历

写法很多，这里只写一种。

使用栈先序遍历二叉树。144. Binary Tree Preorder Traversal .

var preorderTraversal = function(root) {
    if (!root) return []
    const res = []
    const stack = [root]
    while (stack.length) {
        root = stack.pop()
        res.push(root.val)
        if (root.right) stack.push(root.right)
        if (root.left) stack.push(root.left)
    }
    return res
};

使用栈中序遍历二叉树。94. Binary Tree Inorder Traversal .

var inorderTraversal = function(root) {
    const res = []
    const stack = []
    while (root) {
        stack.push(root)
        root = root.left
    }
    while (stack.length) {
        root = stack.pop()
        res.push(root.val)
        root = root.right
        while (root) {
            stack.push(root)
            root = root.left
        }
    }
    return res
};

使用栈后序遍历二叉树.145. Binary Tree Postorder Traversal .

var postorderTraversal = function(root) {
    if (!root) return []
    const res = []
    const stack = [root]
    while (stack.length) {
        const root = stack[stack.length-1]
        if (root.right||root.left) {
            // 设成null是为了防止无限循环
            if (root.right) {
                stack.push(root.right)
                root.right = null
            }
            if (root.left) {
                stack.push(root.left)
                root.left = null
            }
        } else res.push((stack.pop()).val)
    }
    return res
};
// 后序遍历的第二种方法：不改变结点内容
var postorderTraversal = function(root) {
    if (!root) return []
    const res = []
    const stack = [root]
    while (stack.length) {
        root = stack.pop()
        res.unshift(root.val)
        if (root.left) stack.push(root.left)
        if (root.right) stack.push(root.right)
    }
    return res
};