二叉树的递归遍历

前序遍历

var preorderTraversal = function(root) {
    let res = []
    function preorder(cur){
        if(cur == null) return
        res.push(cur.val)
        if(cur.left) preorder(cur.left)
        if(cur.right) preorder(cur.right)

    }
    preorder(root)
    return res
    
};

中序遍历

var inorderTraversal = function(root) {
    let res = []
    function inorder(cur){
        if(cur == null) return 
        if(cur.left) inorder(cur.left)
        res.push(cur.val)
        if(cur.right) inorder(cur.right)
    }
    inorder(root)
    return res
};

后序遍历

var postorderTraversal = function(root) {
    let res = []
    function pestorder(cur){
        if(cur == null) return 
        if(cur.left) pestorder(cur.left)
        if(cur.right) pestorder(cur.right)
        res.push(cur.val)
    }
    pestorder(root)
    return res
};

二叉树的迭代遍历

前序遍历

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

中序遍历

中序遍历和前后序遍历是不同的，前序遍历要访问的元素和要处理的元素顺序是一致的，都是中间节点，但是中序遍历是左中右，先访问的是二叉树顶部的节点，然后一层一层向下访问，直到到达树的最底部，再开始处理节点，这意味着 要访问的元素和要处理的元素顺序是不一致的，所以使用迭代法写中序遍历，需要借助指针的遍历来帮助访问节点，栈则用于处理节点上的元素。

var inorderTraversal = function(root) {
    let stack = []
    let res = []
    let cur = root
    while(stack.length || cur != null){
        if(cur != null){
            stack.push(cur)
            cur = cur.left
        }else {
            cur = stack.pop()
            res.push(cur.val)
            cur = cur.right
        }
    }
    return res
};

后序遍历

var postorderTraversal = function(root) {
    let stack = []
    let res = []
    stack.push(root)
    if(root == null) return res
    while(stack.length){
        let node = stack.pop()
        res.push(node.val)
        if(node.left) stack.push(node.left)
        if(node.right) stack.push(node.right)
    }
    return res.reverse()
};