Question Definition
Given a binary tree, return the zigzag level order traversal of its nodes’ values. (ie, from left to right, then right to left for the next level and alternate between).
For example:
Given binary tree [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its zigzag level order traversal as:
[
[3],
[20,9],
[15,7]
]
Java Solution
public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
Deque<TreeNode> stack = new ArrayDeque<>();
List<List<Integer>> result = new ArrayList<>();
if(root == null) return result;
stack.push(root);
boolean reverse = false;
while(!stack.isEmpty()){
int count = stack.size();
List<Integer> level = new ArrayList<>();
Deque<TreeNode> temp = new ArrayDeque<>();
while(!stack.isEmpty()){
TreeNode top = stack.pop();
if(reverse)
level.add(0, top.val);
else
level.add(top.val);
if(top.left != null)
temp.add(top.left);
if(top.right != null)
temp.add(top.right);
}
stack = temp;
result.add(level);
reverse = !reverse;
}
return result;
}
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