代码随想录算法训练营20期|第十七天|二叉树part04|110.平衡二叉树 ● 257. 二叉树的所有路径 ● 404.左叶子之和

110.平衡二叉树 

递归法：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isBalanced(TreeNode root) {
        if (root == null) return true;
        int left = getHeight(root.left);
        int right = getHeight(root.right);
        if (Math.abs(left - right) > 1) return false;
        return isBalanced(root.left) && isBalanced(root.right);
    }

    private int getHeight(TreeNode root) {
        if (root == null) return 0;
        int left = getHeight(root.left);
        int right = getHeight(root.right);
        int res = Math.max(left, right) + 1;
        return res;
    }
}

迭代法：

257. 二叉树的所有路径 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List binaryTreePaths(TreeNode root) {
        List res = new ArrayList<>();
        if (root == null) return res;
        List path = new ArrayList<>();
        traverse(root, path, res);
        return res;
    }

    private void traverse(TreeNode root, List path, List res) {
        path.add(root.val);
        if (root.left == null && root.right == null) {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < path.size() - 1; i++) {
                sb.append(path.get(i)).append("->");
            }
            sb.append(path.get(path.size() - 1));
            res.add(sb.toString());
            return;
        }
        if (root.left != null) {
            traverse(root.left, path, res);
            path.remove(path.size() - 1);
        }
        if (root.right != null) {
            traverse(root.right, path, res);
            path.remove(path.size() - 1);
        }
    }
}

404.左叶子之和 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        if (root == null) return 0;
        if (root.left == null && root.right == null) return 0;
        int leftSum = sumOfLeftLeaves(root.left);
        if (root.left != null && root.left.left == null && root.left.right == null) {
            leftSum += root.left.val;
        }
        int rightSum = sumOfLeftLeaves(root.right);
        int res = leftSum + rightSum;
        return res;
    }
}

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        if (root == null) return 0;
        int sum = 0;
        Queue queue = new LinkedList<>();
        queue.add(root);

        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode cur = queue.poll();
                if (cur.left != null) {
                    queue.offer(cur.left);
                    if (cur.left.left == null && cur.left.right == null) {
                        sum += cur.left.val;
                    }
                }
                if (cur.right != null) queue.offer(cur.right);
            }
        }
        return sum;
    }
}