代码随想录算法训练营第第14天 | 二叉树递归遍历(递归法、迭代、统一的迭代方法)
递归遍历 (必须掌握)
二叉树的三种递归遍历掌握其规律后,其实很简单
题目链接/文章讲解/视频讲解:https://programmercarl.com/二叉树的递归遍历.html
迭代遍历 (基础不好的录友,迭代法可以放过)
题目链接/文章讲解/视频讲解:https://programmercarl.com/二叉树的迭代遍历.html
统一迭代 (基础不好的录友,迭代法可以放过)
这是统一迭代法的写法, 如果学有余力,可以掌握一下
题目链接/文章讲解:https://programmercarl.com/二叉树的统一迭代法.html
- 二叉树的前序遍历
一、递归写法,主要先理解递归三部曲
function traverse(node,arr) {
if(node==null) return;
arr.push(node.val);
traverse(node.left,arr);
traverse(node.right,arr);
}
/**
* 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) {
if(root==null) return [];
const res = [];
traverse(root,res);
return res;
};
二、迭代写法
/**
* @param {TreeNode} root
* @return {number[]}
*/
var preorderTraversal = function(root) {
if (root==null) return [];
const stack = [root];
const res = [];
while(stack.length){
let cur = stack.pop();
res.push(cur.val);
cur.right && stack.push(cur.right);
cur.left && stack.push(cur.left);
}
return res;
};
- 二叉树的中序遍历
一、递归写法
function traverse(node,arr) {
if(node==null) return;
traverse(node.left,arr);
arr.push(node.val);
traverse(node.right,arr);
}
/**
* 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) {
if(root==null) return [];
const res = [];
traverse(root,res);
return res;
}
二、迭代写法
/**
* @param {TreeNode} root
* @return {number[]}
*/
var inorderTraversal = function(root) {
if (root == null) return [];
const stack = [];
const res = [];
let cur = root;
while(stack.length || cur){
if(cur){
stack.push(cur);
cur = cur.left;
} else {
cur = stack.pop();
res.push(cur.val);
cur = cur.right;
}
}
return res;
};
三、统一的迭代写法
/**
* @param {TreeNode} root
* @return {number[]}
*/
var inorderTraversal = function(root) {
if (root == null) return [];
const stack = [root];
const res = [];
while(stack.length) {
let node = stack[stack.length-1];
if (node !== null) {
stack.pop();
node.right && stack.push(node.right);
stack.push(node);
stack.push(null);
node.left && stack.push(node.left);
} else {
stack.pop();
let cur = stack.pop();
res.push(cur.val);
}
}
return res;
};
145
递归写法和中序先序的递归类似,在此不展示了
迭代写法
/**
* 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) {
if (root == null) return [];
const stack = [root];
const res = [];
while(stack.length){
let cur = stack.pop();
res.push(cur.val);
cur.left && stack.push(cur.left);
cur.right && stack.push(cur.right);
}
return res.reverse();
};
统一的迭代写法
/**
* @param {TreeNode} root
* @return {number[]}
*/
var postorderTraversal = function(root) {
if (root == null) return [];
const res = [];
const stack = [root];
while(stack.length){
let node = stack[stack.length-1];
if (node!==null) {
stack.pop();
stack.push(node);
stack.push(null);
node.right && stack.push(node.right);
node.left && stack.push(node.left);
} else {
stack.pop();
let cur = stack.pop();
res.push(cur.val);
}
}
return res;
};