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Postorder.java
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62 lines (52 loc) · 1.55 KB
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/*
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\
2
/
3
return [3,2,1].
*/
public class Solution { //Iterative
public ArrayList<Integer> postorderTraversal(TreeNode root) {
ArrayList<Integer> result = new ArrayList<Integer>();
Stack<TreeNode> stack = new Stack<TreeNode>();
TreeNode prev = null; // previously traversed node
TreeNode curr = root;
if (root == null) {
return result;
}
stack.push(root);
while (!stack.empty()) { // ???
curr = stack.peek();
if (prev == null || prev.left == curr || prev.right == curr) { // traverse down the tree
if (curr.left != null) { // 先左再右
stack.push(curr.left);
} else if (curr.right != null) {
stack.push(curr.right);
}
} else if (curr.left == prev) { // traverse up the tree from the left
if (curr.right != null) {
stack.push(curr.right);
}
} else { // traverse up the tree from the right
result.add(curr.val);
stack.pop();
}
prev = curr;
}
return result;
}
}
/*
//Recursive
public ArrayList<Integer> postorderTraversal(TreeNode root) {
ArrayList<Integer> result = new ArrayList<Integer>();
if (root == null) {
return result;
}
result.addAll(postorderTraversal(root.left));
result.addAll(postorderTraversal(root.right));
result.add(root.val);
return result;
}
*/