中序遍历

1.递归

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Inorder in ArrayList which contains node values.
     */
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        // write your code here
        ArrayList<Integer> result = new ArrayList<Integer>();
        traverse(root, result);
        return result;
    }
    private void traverse(TreeNode root, ArrayList<Integer> result) {
        if (root == null) {
            return;
        }
        traverse(root.left, result);
        result.add(root.val);
        traverse(root.right, result);
    }
}

与前序遍历区别：改变顺序而已

2.分治，两处错误

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Inorder in ArrayList which contains node values.
     */
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        // write your code here
        ArrayList<Integer> result = new ArrayList<Integer>();
        if (root == null) {
            return result;
        }
        ArrayList<Integer> left = inorderTraversal(root.left);
        ArrayList<Integer> right = inorderTraversal(root.right);
        
        result.addAll(left);
        result.add(root.val);//忘记root.val，写成root
        result.addAll(right); 
        return result; //忘记
    }
 
}

3.非递归的方法，使用栈

先从根节点把所以左边的点入栈，

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Inorder in ArrayList which contains node values.
     */
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        // write your code here
        ArrayList<Integer> result = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode curt = root;
        while (curt != null || !stack.empty()) {
            while (curt != null) {
                stack.add(curt); //stack.push(curt);这两个有啥区别？
                curt = curt.left;
            }
            //curt = stack.peek();
            //stack.pop();
             curt = stack.pop();//这样也行

            result.add(curt.val);
            curt = curt.right;
        }
        return result;
    }
 
}