[算法]死磕二叉树专题算法

Posted 2020-10-30 陈驰字新宇

1. 二叉树遍历（递归和非递归）

构造二叉树：

class Node{
    public String value;
    public Node left;
    public Node right;
    public Node(String value) {
        this.value = value;
    }
}

递归版前序遍历：

public static void preOrder(Node head){
        if(head != null){
            System.out.print(head.value + " ");
            preOrder(head.left);
            preOrder(head.right);
        }
    }

递归版中序遍历：

public static void inOrder(Node head){
        if(head != null){
            inOrder(head.left);
            System.out.print(head.value + " ");
            inOrder(head.right);
        }
    }

递归版后序遍历：

    public static void posOrder(Node head){
        if(head != null){
            posOrder(head.left);
            posOrder(head.right);
            System.out.print(head.value + " ");
        }
    }

非递归版前序遍历：

public static void preOrder(Node head){
        if(head != null){
            Stack<Node> stack = new Stack<>();
            stack.push(head);
            while(!stack.isEmpty()){
                Node pop = stack.pop();
                System.out.print(pop.value + " ");
                if(pop.right != null)
                    stack.push(pop.right);
                if(pop.left != null)
                    stack.push(pop.left);
            }
        }
    }

非递归版中序遍历：

    public static void inOrder(Node head){
        if(head != null){
            Stack<Node> stack = new Stack<>();
            while(!stack.isEmpty() || head != null){
                if(head != null){
                    stack.push(head);
                    head = head.left;
                }else{
                    head = stack.pop();
                    System.out.print(head.value + " ");
                    head = head.right;
                }
            }
        }
    }

非递归版后序遍历：

    public static void postOrder(Node head){
        if(head != null){
            Stack<Node> stack1 = new Stack<>();
            Stack<Node> stack2 = new Stack<>();
            stack1.push(head);
            while(!stack1.isEmpty()){
                Node pop = stack1.pop();
                stack2.push(pop);
                if(pop.left != null){
                    stack1.push(pop.left);
                }
                if(pop.right != null){
                    stack1.push(pop.right);
                }
            }
            while(!stack2.isEmpty()){
                System.out.print(stack2.pop().value + " ");
            }
        }
    }

这里用了两个栈，其实一个栈也能实现，这里这样做是因为可以和前序遍历对比着记，比较容易。