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。
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;
}
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