【Leetcode】1612. Check If Two Expression Trees are Equivalent_leetcode 1612题

题目地址：

https://leetcode.com/problems/check-if-two-expression-trees-are-equivalent/

给定两个只含'+'和英文小写字母作为运算数的表达式树，问这两个表达式树所对应的表达式是否等价。等价的含义是两个表达式作为代数式用等号连起来是恒等式。

只需要数一下两个表达式树所含字母的类型、数量都相同即可。代码如下：

public class Solution {
    public boolean checkEquivalence(Node root1, Node root2) {
        int[] count = new int[26];
        dfs(root1, count, 1);
        dfs(root2, count, -1);
        
        for (int i = 0; i < count.length; i++) {
            if (count[i] != 0) {
                return false;
            }
        }
        
        return true;
    }
    
    private void dfs(Node cur, int[] count, int diff) {
        if (cur == null) {
            return;
        }
        
        if (cur.val != '+') {
            count[cur.val - 'a'] += diff;
        }
        
        dfs(cur.left, count, diff);
        dfs(cur.right, count, diff);
    }
}

class Node {
    char val;
    Node left, right;
    
    public Node(char val) {
        this.val = val;
    }
}
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时间复杂度$O(n)$，空间$O(h)$。