Data Structure: How to traversal Binary Tree Iteratively (Preorder, Inorder and Postorder)

Preorder Traversal: 

public class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<Integer>();
if (root == null) {
return res;
}

LinkedList<TreeNode> stack = new LinkedList<TreeNode>();
TreeNode cur = root;
while (cur != null || !stack.isEmpty()) {
while (cur != null) {
res.add(cur.val);
stack.addFirst(cur);
cur = cur.left;
}

cur = stack.removeFirst();
cur = cur.right;
}

return res;
}
}

Inorder Traversal: 

public class Solution {
public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<Integer>();
if (root == null) {
return res;
}

LinkedList<TreeNode> stack = new LinkedList<TreeNode>();
TreeNode cur = root;

while (cur != null || !stack.isEmpty()) {
while (cur != null) {
stack.addFirst(cur);
cur = cur.left;
}

cur = stack.removeFirst();
res.add(cur.val);
cur = cur.right;
}

return res;
}
}

Postorder Traversal:

public class Solution {
public List<Integer> postorderTraversal(TreeNode root) {
LinkedList<Integer> res = new LinkedList<Integer>();
if (root == null) {
return res;
}

LinkedList<TreeNode> stack = new LinkedList<TreeNode>();
TreeNode cur = root;
while (cur != null || !stack.isEmpty()) {
while (cur != null) {
stack.addFirst(cur);
res.addFirst(cur.val);
cur = cur.right;
}

cur = stack.removeFirst();
cur = cur.left;
}
return res;
}
}

总结而言，就是前序和后序遍历都是在里面的while循环里面加。两者的不同是add与addFirst，以及先left后right，或者先right后left。只有中序遍历不需要在里面的while循环给答案加元素。