public classPreOrderBT {
publicArrayListresult=newArrayList<>();
publicArrayListpre=newArrayList<>();
public voidpreOrder(TreeNode root){
if(root ==null){
return;
}
//访问根->左孩子->右孩子
result.add(root.val);
preOrder(root.left);
preOrder(root.right);
}
// no recursion
public voidpreOrderTraversal(TreeNode root)
{
if(root ==null){
return;
}
Stack stack =newStack();
stack.push(root);
while(!stack.isEmpty()){
TreeNode node = stack.pop();
pre.add(node.val);
if(node.right!=null){
stack.push(node.right);
}
if(node.left!=null){
stack.push(node.left);
}
}
}
}
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