一篇文章教你如何用多种迭代写法实现二叉树遍历

一篇文章教你如何用多种迭代写法实现二叉树遍历

目录思想实现总结

思想

利用栈和队列都可以实现树的迭代遍历。递归的写法将这个遍历的过程交给系统的堆栈去实现了，所以思想都是一样的、无非就是插入值的时机不一样。利用栈的先进先出的特点，对于前序遍历、我们可以先将当前的值放进结果集中，表示的是根节点的值、然后将当前的节点加入到栈中、当前的节点等于自己的left、再次循环的时候、也会将left作为新的节点、直到节点为空、也就是走到了树的最左边、然后回退、也就是弹栈、、也可以认为回退的过程是从低向上的、具体就是让当前的节点等于栈弹出的right、继续重复上面的过程，也就实现了树的前序遍历、也就是bfs.后续遍历、中序遍历思想也是类似的。

实现

public List

List res = new Arhttp://rayList<>();

Stack 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 preorderTraversal2(TreeNode root) {

List res = new ArrayList<>();

Stack 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 preorderTraversal3(TreeNode root) {

List res = new ArrayList<>();

if (root == null) return res;

Stack 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 preorderTraversal4(TreeNode root) {

List res = new ArrayList<>();

if (root == null) {

return res;

}

LinkedList 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 inorderTraversal1(TreeNode root) {

List res = new ArrayList<>();

Stack 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 inorderTraversal2(TreeNode root) {

List res = new ArrayList<>();

Stack 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 postorderTraversal1(TreeNode root) {

List res = new ArrayList<>();

if (root == null) return res;

Stack stack = new Stack<>();

stack.push(root);

while (!stack.isEmpty()) {

TreeNode cur = stack.pop();

res.add(cur.val);

if (cur.left != null) {

CqKyvZvjEG stack.push(cur.left);

}

if (cur.right != null) {

stack.push(cur.right);

}

}

Collections.reverse(res);

return res;

}

public List postorderTraversal2(TreeNode root) {

List res = new ArrayList<>();

Stack 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> levelOrder(TreeNode root) {

List> ret = new ArrayList<>();

if(root == null)return ret;

Queue queue = new LinkedList<>();

queue.offer(root);

while (!queue.isEmpty()){

int size = queue.size();

List 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;

}

总结

本篇文章就到这里了，希望能给你带来帮助，也希望您能够多多关注我们的更多内容！

List res = new Arhttp://rayList<>();

Stack 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 preorderTraversal2(TreeNode root) {

List res = new ArrayList<>();

Stack 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 preorderTraversal3(TreeNode root) {

List res = new ArrayList<>();

if (root == null) return res;

Stack 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 preorderTraversal4(TreeNode root) {

List res = new ArrayList<>();

if (root == null) {

return res;

}

LinkedList 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 inorderTraversal1(TreeNode root) {

List res = new ArrayList<>();

Stack 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 inorderTraversal2(TreeNode root) {

List res = new ArrayList<>();

Stack 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 postorderTraversal1(TreeNode root) {

List res = new ArrayList<>();

if (root == null) return res;

Stack stack = new Stack<>();

stack.push(root);

while (!stack.isEmpty()) {

TreeNode cur = stack.pop();

res.add(cur.val);

if (cur.left != null) {

CqKyvZvjEG stack.push(cur.left);

}

if (cur.right != null) {

stack.push(cur.right);

}

}

Collections.reverse(res);

return res;

}

public List postorderTraversal2(TreeNode root) {

List res = new ArrayList<>();

Stack 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> levelOrder(TreeNode root) {

List> ret = new ArrayList<>();

if(root == null)return ret;

Queue queue = new LinkedList<>();

queue.offer(root);

while (!queue.isEmpty()){

int size = queue.size();

List 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;

}

总结

本篇文章就到这里了，希望能给你带来帮助，也希望您能够多多关注我们的更多内容！

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