# 110.balanced-binary-tree

## Statement

• Difficulty: Easy
• Tag: `树` `深度优先搜索` `二叉树`

``````输入：root = [3,9,20,null,null,15,7]

``````

``````输入：root = [1,2,2,3,3,null,null,4,4]

``````

``````输入：root = []

``````

• 树中的节点数在范围 `[0, 5000]`
• `-104 <= Node.val <= 104`

• Link: Balanced Binary Tree
• Difficulty: Easy
• Tag: `Tree` `Depth-First Search` `Binary Tree`

Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as:

a binary tree in which the left and right subtrees of every node differ in height by no more than 1.

Example 1:

``````Input: root = [3,9,20,null,null,15,7]
Output: true
``````

Example 2:

``````Input: root = [1,2,2,3,3,null,null,4,4]
Output: false
``````

Example 3:

``````Input: root = []
Output: true
``````

Constraints:

• The number of nodes in the tree is in the range `[0, 5000]`.
• `-104 <= Node.val <= 104`

## Solution

``````#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

#define endl "\n"
#define fi first
#define se second
#define all(x) begin(x), end(x)
#define rall rbegin(a), rend(a)
#define bitcnt(x) (__builtin_popcountll(x))
#define complete_unique(a) a.erase(unique(begin(a), end(a)), end(a))
#define mst(x, a) memset(x, a, sizeof(x))
#define MP make_pair

using ll = long long;
using ull = unsigned long long;
using db = double;
using ld = long double;
using VLL = std::vector<ll>;
using VI = std::vector<int>;
using PII = std::pair<int, int>;
using PLL = std::pair<ll, ll>;

using namespace __gnu_pbds;
using namespace std;
template <typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
const ll mod = 1e9 + 7;

template <typename T, typename S>
inline bool chmax(T &a, const S &b) {
return a < b ? a = b, 1 : 0;
}

template <typename T, typename S>
inline bool chmin(T &a, const S &b) {
return a > b ? a = b, 1 : 0;
}

#ifdef LOCAL
#include <debug.hpp>
#else
#define dbg(...)
#endif

#ifdef LOCAL

struct TreeNode {
int val;
TreeNode *left;
TreeNode *right;
TreeNode() : val(0), left(nullptr), right(nullptr) {}
TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};

#endif

class Solution {
public:
bool isBalanced(TreeNode *root) {
bool res = true;
auto dfs = [&](auto self, TreeNode *rt) -> int {
if (!rt) {
return 0;
}

int l = self(self, rt->left);
int r = self(self, rt->right);

if (abs(l - r) > 1) {
res = false;
}

return max(l, r) + 1;
};

dfs(dfs, root);
return res;
}
};

#ifdef LOCAL

int main() {
return 0;
}

#endif
``````