• 二叉树具有天然的递归结构

LeetCode 965. 单值二叉树

class Solution {public boolean isUnivalTree(TreeNode root) {// 节点数范围，可直接取根节点值int val = root.val;return dfs(root, val);}public boolean dfs(TreeNode node, int val) {if (node == null)return true;if (node.val != val)return false;return dfs(node.left, val) && dfs(node.right, val);}
}

LeetCode 257. 二叉树的所有路径

class Solution {public List<String> binaryTreePaths(TreeNode root) {List<String> ans = new ArrayList<String>();dfs(ans, "", root);return ans;}public void dfs(List<String> ans, String path, TreeNode node) {if (node != null) {StringBuffer pathSB = new StringBuffer(path);pathSB.append(Integer.toString(node.val));// 叶子节点if (node.left == null && node.right == null)ans.add(pathSB.toString());else {pathSB.append("->");dfs(ans, pathSB.toString(), node.left);dfs(ans, pathSB.toString(), node.right);}}}
}

LeetCode 113. 路经总和II

class Solution {List<List<Integer>> ans = new LinkedList<List<Integer>>();Deque<Integer> path = new LinkedList<Integer>();public List<List<Integer>> pathSum(TreeNode root, int targetSum) {if (root == null)return ans;dfs(root, targetSum);return ans;}public void dfs(TreeNode node, int targetSum) {if (node == null)return;path.offerLast(node.val);targetSum -= node.val;if (node.left == null && node.right == null && targetSum == 0)ans.add(new LinkedList<Integer>(path));dfs(node.left, targetSum);dfs(node.right, targetSum);path.pollLast();}
}

LeetCode 563. 二叉树的坡度

class Solution {int ans;public int findTilt(TreeNode root) {ans = 0;dfs(root);return ans;}public int dfs(TreeNode node) {if (node == null)return 0;int left_sum = dfs(node.left);int right_sum = dfs(node.right);// 整个树 的坡度就是其所有节点的坡度之和ans += Math.abs(left_sum - right_sum);return left_sum + right_sum + node.val;}
}

LeetCode 687. 最长同值路径

class Solution {int ans;public int longestUnivaluePath(TreeNode root) {ans = 0;if (root == null)return 0;dfs(root);return ans;}public int dfs(TreeNode node) {if (node == null)return 0;int left = dfs(node.left);int right = dfs(node.right);if (node.left != null && node.left.val == node.val)left = left + 1;elseleft = 0;if (node.right != null && node.right.val == node.val)right = right + 1;elseright = 0;ans = Math.max(ans, left+right);return Math.max(left, right);}
}

LeetCode 124. 二叉树中的最大路径和

class Solution {int ans;public int maxPathSum(TreeNode root) {ans = Integer.MIN_VALUE;dfs(root);return ans;}public int dfs(TreeNode node) {if (node == null)return 0;int left = Math.max(dfs(node.left), 0);int right = Math.max(dfs(node.right), 0);ans = Math.max(ans, node.val+left+right);return node.val + Math.max(left, right);}
}