二叉樹的非遞歸遍歷

本來遍歷用遞歸是一個非常簡單的事情，但是偏偏某書喜歡用非遞歸寫，真傷腦筋。剛開始的時候，覺得不就是把函數換成棧麼，簡單吶。結果，換了一天硬是沒有寫出來。抄書上的代碼，居然沒看懂。真是服了。後來從簡書上找到一篇文章，得到了一個非常簡單的非遞歸寫法，當然，簡單是有代價的。
原代碼用C++寫的，這裏換成Java。
使用一個輔助的Pair相當巧妙的簡化了代碼

幾種遍歷的代碼

    public static void preOrderNonRecursive_v3(BinaryTreeNode root) {
        if (root == null) return;
        Stack<Pair<BinaryTreeNode, Boolean>> stack = new Stack<>();
        stack.push(new Pair<>(root, false));
        boolean visited;
        while(!stack.isEmpty()){
            root = stack.peek().first;
            visited = stack.peek().second;
            stack.pop();
            if(root==null) continue;
            if(visited) System.out.print(root.data+" ");
            else{
                stack.push(new Pair<>(root.right,false));
                stack.push(new Pair<>(root.left,false));
                stack.push(new Pair<>(root, true));
            }
        }
    }
        public static void inOrderNonRecursive_v3(BinaryTreeNode root) {
        if (root == null) return;
        Stack<Pair<BinaryTreeNode, Boolean>> stack = new Stack<>();
        stack.push(new Pair<>(root, false));
        boolean visited;
        while(!stack.isEmpty()){
            root = stack.peek().first;
            visited = stack.peek().second;
            stack.pop();
            if(root==null) continue;
            if(visited) System.out.print(root.data+" ");
            else{
                stack.push(new Pair<>(root.right,false));
                stack.push(new Pair<>(root, true));
                stack.push(new Pair<>(root.left,false));
            }
        }
    }
        public static void postOrderNonRecursive_v3(BinaryTreeNode root) {
        if (root == null) return;
        Stack<Pair<BinaryTreeNode, Boolean>> stack = new Stack<>();
        stack.push(new Pair<>(root, false));
        boolean visited;
        while(!stack.isEmpty()){
            root = stack.peek().first;
            visited = stack.peek().second;
            stack.pop();
            if(root==null) continue;
            if(visited) System.out.print(root.data+" ");
            else{
                stack.push(new Pair<>(root, true));
                stack.push(new Pair<>(root.right,false));
                stack.push(new Pair<>(root.left,false));
            }
        }
    }

Pair的結構

    private static class Pair<T,U>{
        T first;
        U second;

        public Pair(T first, U second) {
            this.first = first;
            this.second = second;
        }
    }

簡單的TreeNode

public class BinaryTreeNode {
    private int data;
    private BinaryTreeNode left;
    private BinaryTreeNode right;
    public BinaryTreeNode(int data) {
        this.data = data;
    }

    public BinaryTreeNode(int data, BinaryTreeNode left, BinaryTreeNode right) {
        this.data = data;
        this.left = left;
        this.right = right;
    }
    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        builder.append(data)
                .append("(")
                .append(left == null ? null : left.data)
                .append(",")
                .append(right == null ? null : right.data)
                .append(")");
        return builder.toString();
    }