二叉树的后序遍历（非递归算法）

• 给定一个二叉树，返回它的 后序 遍历。

typedef struct TreeNode* STDataType;
typedef struct Stack
{
	STDataType* _a;
	int _top; // 栈顶
	int _capacity; // 容量
}Stack;
Stack ps;


void StackInit(Stack* ps)
{
	assert(ps);
	ps->_a = NULL;
	ps->_top = 0;
	ps->_capacity = 0;
	
}

void StackDestory(Stack* ps)
{
	assert(ps);
	free(ps->_a);//
	ps->_a = NULL;
	ps->_top = 0;
	ps->_capacity = 0;
}

void StackPush(Stack* ps, STDataType x)
{
	assert(ps);
	if (ps->_top == ps->_capacity)
	{
		int newcapacity = ps->_capacity==0?4:2 * (ps->_capacity);//
		ps->_a= (STDataType*)realloc(ps->_a, newcapacity*sizeof(STDataType));
		assert(ps->_a != NULL);
		ps->_capacity = newcapacity;
	}

	ps->_a[ps->_top] = x;
	++(ps->_top);

}

void StackPop(Stack* ps)
{
	assert(ps&&ps->_top > 0);

	--(ps->_top);
}


STDataType StackTop(Stack* ps)
{
	assert(ps&&ps->_top > 0);
	return  ps->_a[ps->_top-1];

}
	
int StackEmpty(Stack* ps)
{
	assert(ps);
	if (ps->_top == 0)
	{
		return 0;
	}
	else
	{
		return 1;
	}
}

int StackSize(Stack* ps)
{
	assert(ps);
	return ps->_top;
}

int* postorderTraversal(struct TreeNode* root, int* returnSize){
  Stack st;
  StackInit(&st);
  struct TreeNode* top = StackTop(&st);
  struct TreeNode* prev = NULL;
while (StackEmpty(&st)!=0 || cur!=NULL){
  while (cur!=NULL){
    StackPush(&st);
    (*returnSize)++;
    cur = cur->left;
  }
  struct TreeNode* top = StackTop(&st);
  if (top->right==prev || top->right==NULL){
    StackPop(&st);
    prev = top;
  }
  else{
    cur = cur->right;
  }
int *array = (int*)malloc(sizeof(int)*(*returnSize));
int i = 0;
cur = root;
  while (StackEmpty(&st)!=0 || cur!=NULL){
    while (cur!=NULL){
      StackPush(&st,cur);
      cur = cur->left;
    }
  struct TreeNode* top = StackTop(&st);
   if (top->right==prev || top->right==NULL){
  //当前结点的右子树已经访问过了。
    array[i++] = top->val;
    prev = top;//把当前结点作为新的上一个结点，然后访问下一个结点
    StackPop(&st);
   }
   else{
    cur = top->right;
   }
  return array;
 }