二叉树学习——前序、中序、后序遍历（Java实现）

递归实现:

public static ArrayList<Integer> preorderTraversal(TreeNode root) {
ArrayList<Integer> sort = new ArrayList<>();
if (root == null) {
return sort;
}
if (root.left == null && root.right == null) {
sort.add(root.val);
return sort;
}
sort.add(root.val);
sort.addAll(preorderTraversal(root.left));
sort.addAll(preorderTraversal(root.right));
return sort;
}

public static ArrayList<Integer> inorderTraversal(TreeNode root) {
ArrayList<Integer> sort = new ArrayList<>();
if (root == null) {
return sort;
}
if (root.left == null && root.right == null) {
sort.add(root.val);
return sort;
}
sort.addAll(inorderTraversal(root.left));
sort.add(root.val);
sort.addAll(inorderTraversal(root.right));
return sort;
}

public static ArrayList<Integer> postorderTraversal(TreeNode root) {
ArrayList<Integer> sort = new ArrayList<>();
if (root == null) {
return sort;
}
if (root.left == null && root.right == null) {
sort.add(root.val);
return sort;
}
sort.addAll(postorderTraversal(root.left));
sort.addAll(postorderTraversal(root.right));
sort.add(root.val);
return sort;
}

非递归实现

public static ArrayList<Integer> preorderTraversal(TreeNode root) {
ArrayList<Integer> sort = new ArrayList<>();
TreeNode cur = root;
Stack<TreeNode> stack = new Stack<>();
while (cur != null || !stack.isEmpty()) {
while (cur != null) {
sort.add(cur.val);
stack.push(cur);
cur = cur.left;
}
cur = stack.pop();
cur = cur.right;
}
return sort;
}

public static ArrayList<Integer> inorderTraversal(TreeNode root) {
ArrayList<Integer> sort = new ArrayList<>();
TreeNode cur = root;
Stack<TreeNode> stack = new Stack<>();
while (cur != null || !stack.isEmpty()) {
while (cur != null) {
stack.push(cur);
cur = cur.left;
}
cur = stack.pop();
sort.add(cur.val);
cur = cur.right;
}
return sort;
}

public static ArrayList<Integer> postorderTraversal(TreeNode root) {
ArrayList<Integer> sort = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
Stack<TreeNode> stack2 = new Stack<>();
TreeNode cur = root;
while (cur != null || !stack.isEmpty()) {
while (cur != null) {
stack.push(cur);
stack2.push(cur);
cur = cur.right;
}
cur = stack.pop();
cur = cur.left;
}
sort.add(stack2.pop().val);
while (stack2.size() > 0) {
cur = stack2.pop();
sort.add(cur.val);
}
return sort;
}