代码随想录算法训练营第11天 | 20. 有效的括号 1047. 删除字符串中的所有相邻重复项 150. 逆波兰表达式求值

20. 有效的括号

class Solution {
public:
    bool isValid(string s) {
        stack st;
        if (s.size() % 2 != 0) return false;
        for (int i = 0; i < s.size(); i++) {
            if (s[i] == '(') st.push(')');
            else if (s[i] == '[') st.push(']');
            else if (s[i] == '{') st.push('}');
            else if (st.empty() || s[i] != st.top()) return false;
            else st.pop();
        }
        return st.empty();
    }
};

1047. 删除字符串中的所有相邻重复项

class Solution {
public:
    string removeDuplicates(string s) {
        string result;
        for (char c : s) {
            if (result.empty() || c != result.back()) result.push_back(c);
            else result.pop_back();
        }
        return result;
    }
};

150. 逆波兰表达式求值

class Solution {
public:
    int evalRPN(vector& tokens) {
        stack result;
        for (int i = 0; i < tokens.size(); i++) {
            if (tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/") {
                int nums1 = result.top();
                result.pop();
                int nums2 = result.top();
                result.pop();
                if (tokens[i] == "+") result.push(nums1 + nums2);
                if (tokens[i] == "-") result.push(nums2 - nums1);
                if (tokens[i] == "*") result.push(nums1 * nums2);
                if (tokens[i] == "/") result.push(nums2 / nums1);
            } else {
                result.push(stoll(tokens[i]));
            }
        }
        return result.top();
    }
};