2020-12-09 First, Middle and Second Order Traversal of Binary Tree

Recursive version

Pre-order traversal algorithm formula

• Visit the root node
• Preorder traversal of the left subtree
• Pre-order traversal of the right subtree

const preorder = (root) => {
      if(!root) { return }
      console.log(root.val)
      preorder(root.left)
      preorder(root.right)
}

In-order traversal algorithm formula

• In-order traversal  of the left subtree of the root node
• Access root node
• In-order traversal of the right subtree with a few points

const inorder = (root) =>{
    if(!root) {return}
    inorder(roor.left)
    console.log(root.val)
    inorder(roor.right)

}

Post-order traversal algorithm formula

• Post-order traversal of the left subtree of the root node
• Post-order traversal of the right subtree of the root node
• Visit the root node

const postorder = ( root ) => {
   if(!root) { return}
   postorder（root.left）
   postorder（root.right）
   console.log(root.val)
} 

 

Non-recursive version

 

Preorder traversal algorithm

• Visit the root node
• Preorder traversal of the left subtree
• Pre-order traversal of the right subtree

const preorder = (root) => {
     if(!root){ return}
     const stack = [root]// 栈
     while(stack.length){ 
        const n = stack.pop() // 出栈
        console.log(n)
        //  后进先出特性 所以先right 先入栈
        if(stack.right) stack.push（stack.right）
        if(stack.left) stack.push（stack.left）
     }
}

In-order traversal

• In-order traversal  of the left subtree of the root node
• Access root node
• In order to traverse the right subtree of a few points ,

const inorder = (root) => {
      if(!root) { return}
      const stack = [] ;
      let p  = root ;  // 声明一个指针
      while(stack.length || p){ // stack 有值 或者 p 不为空 一直循环
         while(p){
            stack.push(p) // 入栈
            p = p.left ; // 指针指向左节点
         }
         const n = stack.pop() // 出栈
         console.log(n.val) // 访问根节点
         p = n.right // 指针指向右节点
      }
}

Post-order traversal algorithm formula

• Post-order traversal of the left subtree of the root node
• Post-order traversal of the right subtree of the root node
• Visit the root node

const postorder = ( root ) {
    if(!root) { rturn}
     const stack = [root] // 栈 
     const outputStack = [] // outputStack 栈  存储stack栈的反序
    while(stack.length) {
       const n  = stack.pop();  // 出栈 
       outputStack.push(n)  // 存档到outputStack栈中
       if(n.left) stack.push(n.left)
       if(n.left) stack.push(n.right)
    }
    while(outputStack.length){
         const n = outputStack.pop() // 从 outputStack  出栈
        console.log(n.val) // 访问节点
    }
}