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一篇文章教你如何用多種迭代寫法實現(xiàn)二叉樹遍歷

更新時間：2021年08月02日 15:40:39 作者：保護眼睛

這篇文章主要介紹了C語言實現(xiàn)二叉樹遍歷的迭代算法,包括二叉樹的中序遍歷、先序遍歷及后序遍歷等,是非常經(jīng)典的算法,需要的朋友可以參考下

思想

利用棧和隊列都可以實現(xiàn)樹的迭代遍歷。遞歸的寫法將這個遍歷的過程交給系統(tǒng)的堆棧去實現(xiàn)了，所以思想都是一樣的、無非就是插入值的時機不一樣。利用棧的先進先出的特點，對于前序遍歷、我們可以先將當(dāng)前的值放進結(jié)果集中，表示的是根節(jié)點的值、然后將當(dāng)前的節(jié)點加入到棧中、當(dāng)前的節(jié)點等于自己的left、再次循環(huán)的時候、也會將left作為新的節(jié)點、直到節(jié)點為空、也就是走到了樹的最左邊、然后回退、也就是彈棧、、也可以認為回退的過程是從低向上的、具體就是讓當(dāng)前的節(jié)點等于棧彈出的right、繼續(xù)重復(fù)上面的過程，也就實現(xiàn)了樹的前序遍歷、也就是bfs.后續(xù)遍歷、中序遍歷思想也是類似的。

實現(xiàn)

    public List<Integer> preorderTraversal1(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while (!stack.isEmpty() || root != null) {
            while (root != null) {
                res.add(root.val);
                stack.add(root);
                root = root.left;
            }
            TreeNode cur = stack.pop();
            root = cur.right;
        }
        return res;
    }
    public List<Integer> preorderTraversal2(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while (!stack.isEmpty() || root != null) {
            if (root != null) {
                res.add(root.val);
                stack.add(root);
                root = root.left;
            } else {
                TreeNode cur = stack.pop();
                root = cur.right;
            }
        }
        return res;
    }
    public List<Integer> preorderTraversal3(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) return res;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            res.add(cur.val);
            if (cur.right != null) {
                stack.push(cur.right);
            }
            if (cur.left != null) {
                stack.push(cur.left);
            }
        }
        return res;
    }
    public List<Integer> preorderTraversal4(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            root = queue.poll();
            res.add(root.val);
            if (root.right != null) {
                queue.addFirst(root.right);
            }
            if (root.left != null) {
                root = root.left;
                while (root != null) {
                    res.add(root.val);
                    if (root.right != null) {
                        queue.addFirst(root.right);
                    }
                    root = root.left;
                }
            }
        }
        return res;
    }
    public List<Integer> inorderTraversal1(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while (root != null || !stack.isEmpty()) {
            if (root != null) {
                stack.add(root);
                root = root.left;
            } else {
                TreeNode cur = stack.pop();
                res.add(cur.val);
                root = cur.right;
            }
        }
        return res;
    }
    public List<Integer> inorderTraversal2(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.add(root);
                root = root.left;
            }
            TreeNode cur = stack.pop();
            res.add(cur.val);
            root = cur.right;
        }
        return res;
    }
    public List<Integer> postorderTraversal1(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) return res;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            res.add(cur.val);
            if (cur.left != null) {
                stack.push(cur.left);
            }
            if (cur.right != null) {
                stack.push(cur.right);
            }
        }
        Collections.reverse(res);
        return res;
    }
    public List<Integer> postorderTraversal2(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while (!stack.isEmpty()) {
            while (root != null) {
                res.add(root.val);
                stack.push(root);
                root = root.right;
            }
            TreeNode cur = stack.pop();
            root = cur.left;
        }
        Collections.reverse(res);
        return res;
    }
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> ret = new ArrayList<>();
        if(root == null)return ret;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()){
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            while(size!=0){
                TreeNode cur = queue.poll();
                list.add(cur.val);
                if(cur.left!=null){
                    queue.offer(cur.left);
                }
                if(cur.right!= null){
                    queue.offer(cur.right);
                }
                size --;
            }
            ret.add(list);
        }
        return ret;
    }