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BinaryTreeInorderTraversal.java
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BinaryTreeInorderTraversal.java
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import java.util.ArrayList;
import java.util.List;
import java.util.Stack;
/**
* 94. Binary Tree Inorder Traversal
* @author LBW
*/
public class BinaryTreeInorderTraversal {
/**
* Method 1: Recursion. 递归解法. 较为简单
*/
public List<Integer> inorderTraversal(TreeNode root) {
ArrayList<Integer> result = new ArrayList<>();
dfs(root, result);
return result;
}
private void dfs(TreeNode root, ArrayList<Integer> result) {
if (root != null) {
dfs(root.left, result);
result.add(root.val);
dfs(root.right, result);
}
}
/**
* Method 2: Iteration 迭代解法. 模拟递归调用栈
* @param root
* @return
*/
public List<Integer> inorderTraversalTwo(TreeNode root) {
ArrayList<Integer> result = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
TreeNode cur = root;
while (cur != null || !stack.isEmpty()) { // 循环条件:cur不为空 或者 栈不为空。 其中cur不为空的情况出现在:左节点和根节点都访问完毕时,当前指向根节点的右节点时,栈恰好为空,但是此时还未结束。
if (cur != null) { // 当cur不为空时,将cur加入栈中,并指向左节点.
stack.push(cur);
cur = cur.left;
}
else { // 当cur为空时,从栈中弹出顶部元素,加入结果中,并将cur指向其右节点。
cur = stack.pop();
result.add(cur.val);
cur = cur.right;
}
}
return result;
}
public static void main(String[] args) {
BinaryTreeInorderTraversal binaryTreeInorderTraversal = new BinaryTreeInorderTraversal();
TreeNode root = BinaryTreePostorderTraversal.getTreeA();
System.out.println(binaryTreeInorderTraversal.inorderTraversalTwo(root));
}
}