二叉树的先序、中序、后序遍历
class TreeNode {
constructor(val) {
this.val = val;
this.left = this.right = null;
}
}
先中后序遍历(递归实现)
let preOrder = function(root){
if(!root)
return [];
return [root.val].concat(preOrder(root.left), preOrder(root.right));
}
let midOrder = function(root){
if (!root)
return [];
return midOrder(root.left).concat([root.val], midOrder(root.right));
}
let afterOrder = function(root) {
if (!root)
return [];
return afterOrder(root.left).concat(afterOrder(root.right), [root.val]);
}
先中后序遍历(非递归,通过栈实现)
// 先序遍历1
let preOrder1 = function(root){
let p = root,
stack = [],
res = []
while (p != null || stack.length) {
while (p != null) {
stack.push(p)
res.push(p.val)
p = p.left
}
p = stack.pop()
p = p.right
}
return res
}
// 先序遍历2
var preOrder2 = function(root) {
let stack = [],p = root,res = []
if(!p) return res
stack.push(p)
while(stack.length){
p = stack.pop()
res.push(p.val)
if(p.right) stack.push(p.right)
if(p.left) stack.push(p.left)
}
return res
}
// 中序遍历
let midOrder = function(root){
let p = root,
stack = [],
res = []
while (p != null || stack.length) {
while (p != null) {
stack.push(p)
p = p.left
}
p = stack.pop()
res.push(p.val)
p = p.right
}
return res
}
// 后序遍历
let afterOrder = function(root) {
let p = root,
stack = [],
res = [],
last = null
while (p != null || stack.length) {
while (p != null) {
stack.push(p)
p = p.left
}
p = stack[stack.length - 1]
if (p.right && last != p.right)
p = p.right
else if (p.right == null || p.right == last) {
res.push(p.val)
last = p
stack.pop()
p = null
}
}
return res
}