刚学完树的迭代遍历，但是不一样的写法，会导致人大佬迷糊记不清，所以在此纪录一下统一的写法
里面的方法使用到的是空标记法，单判断到空字节，则证明需要处理空节点了
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| /**
* 前序遍历
* 中、左、右
*/
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> result = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
//栈不为空
while (!stack.isEmpty()) {
TreeNode node = stack.peek();
if (node != null) {
stack.pop();
stack.push(node.rigth);//右
stack.push(node.left);//左
stack.push(node); //中
stack.push(null);
} else {
stack.pop();//弹出空元素
node = stack.peek();
if (node != null) {
result.add(node.val);
stack.pop();
}
}
}
return result;
}
/**
* 后续遍历
* 左、右、中
*/
public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> result = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
//栈不为空
while (!stack.isEmpty()) {
TreeNode node = stack.peek();
if (node != null) {
stack.pop();
stack.push(node); //中
stack.push(null);
stack.push(node.rigth);//右
stack.push(node.left);//左
} else {
stack.pop();//弹出空元素
node = stack.peek();
if (node != null) {
result.add(node.val);
stack.pop();
}
}
}
return result;
}
/**
* 中序遍历
* 左、中、右
*/
public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> result = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
//栈不为空
while (!stack.isEmpty()) {
TreeNode node = stack.peek();
if (node != null) {
stack.pop();
stack.push(node.rigth);//右
stack.push(node); //中
stack.push(null);
stack.push(node.left);//左
} else {
stack.pop();//弹出空元素
node = stack.peek();
if (node != null) {
result.add(node.val);
stack.pop();
}
}
}
return result;
}
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