二叉树的迭代遍历

前序

使用栈数据结构存储节点，先遍历当前节点，然后push右节点，再push左节点（保证弹出顺序是先左后右）

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var preorderTraversal = function (root) {
    let result = []
    if (root === null) {
        return result
    }
    let stack = []
    stack.push(root)
    while (stack.length) {
        const curNode = stack.pop()
        result.push(curNode.val)
        if (curNode.right) stack.push(curNode.right)
        if (curNode.left) stack.push(curNode.left)
    }
    return result
};

后序

145.二叉树的后序遍历

使用栈数据结构存储节点，先遍历当前节点，然后push左节点，再push右节点（保证弹出顺序是先右后左），此时遍历顺序是中右左，然后反转数组，得到左右中的遍历顺序，即后序遍历

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var postorderTraversal = function (root) {
    let result = []
    if (root == null) {
        return result
    }
    let stack = [root]
    while (stack.length) {
        const top = stack.pop()
        result.push(top.val)
        if (top.left) stack.push(top.left)
        if (top.right) stack.push(top.right)
    }
    result.reverse()
    return result
};

中序

94.二叉树的中序遍历

因为要访问的元素和要处理的元素顺序是不一致的，在使用迭代法写中序遍历，就需要借用指针的遍历来帮助访问节点，栈则用来处理节点上的元素。

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var inorderTraversal = function (root) {
    let result = []
    if (root === null) return result
    let stack = []
    let cur = root
    while (cur !== null || stack.length) {
        if (cur !== null) {
            stack.push(cur)
            cur = cur.left
        } else {
            cur = stack.pop()
            result.push(cur.val)
            cur = cur.right
        }
    }
    return result
};

二叉树的迭代遍历 ​

前序 ​

后序 ​

中序 ​

二叉树的迭代遍历

前序

后序

中序