树遍历非递归

非递归的遍历模板

1.先序遍历 根左右

Stack<TreeNode> stack = new Stack<>();
while(!stack.isEmpty() || root != null){
  while(root != null){
           visit(根节点)
     stack.push(root);
     root = root.left;
  }
  root = stack.pop();
  root = root.right;
}

2.中序遍历 左根右

Stack<TreeNode> stack = new Stack<>();
while(!stack.isEmpty() || root != null){
  while(root != null){
    stack.push(root);
    root = root.left;
  }
  root = stack.pop();
  visit(根节点)
  root = root.right;
}

3.后序遍历 左右根

3.1 如果只是希望获得后序遍历结果

//观察可知 后序遍历为 左右根 ,逆序是 根右左,而先序遍历是 根左右
//所以只需要简单的改造先序遍历,使其变为根右左
//然后将访问结果reverse即为后序遍历结果
Stack<TreeNode> stack = new Stack<>();
List<Integer> list = new ArrayList<>();
while (!stack.isEmpty() || root != null) {
  while (root != null) {
    list.add(root.val);
    stack.push(root);
    root = root.right;
  }
  root = stack.pop();
  root = root.left;
}
Collections.reverse(list);

3.2 若不仅仅只是想获得后序遍历结果,而是每次都需要进行一些处理

Stack<TreeNode> stack = new Stack<>();
List<Integer> list = new ArrayList();
TreeNode lastVisitedNode = null;
while (!stack.isEmpty() || root != null) {
  while (root != null) {
    stack.push(root);
    root = root.left;
  }
  TreeNode topNode = stack.peek();
  if (topNode.right != null && topNode.right != lastVisitedNode) root = topNode.right;
  else {
    stack.pop();
    //visit(node) -- list.add(topNode.val);
    lastVisitedNode = topNode;
  }
}
return list