樹的遍歷 非遞歸實現

1.從左到右，從上到下的廣度優先遍歷（用隊列實現）：

void breadthFirst(TreeNode *root) {
  Queue<TreeNode *>  queue;
  TreeNode * p = root;
  if(p != 0) {
    queue.push(p);
    while(!queue.empty()) {
       p = queue.front();
       queue.pop();
       visit(p);
       if(p->left !=0) {
            queue.push(p->left);  
       }
       if(p->right !=0) {
          queue.push(p->right);
       }
     }
  }
}

2.前序遍歷的非遞歸實現

void iterativePreorder(TreeNode * root) {
stack <TreeNode *> travStack;
TreeNode *p = root;
if(p != 0) {
travStack.push(p);
while(!travStack.empty()) {
p = travStack.pop();
visit(p);
if(p->right != 0) {
travStack.push(p->right);
}
if(p->left != 0) {
travStack.push(p->left);
}
}
}
}

3.後序遍歷的非遞歸實現

void iterativePostorder(TreeNode *root) {
stack<TreeNode *> travStack;
TreeNode * p = root,*q=root;
while(p !=0 ) {
for(;p->right!=0; p=p->left) {
travStack.push(p);
}
while(p!=0 && (p->right == 0 || p->right == q)) {
visit(p);
q = p;
if(travStack.empty())
return;
p = travStack.pop();
}
travStack.push(p);
p=p->right;
}
}