LeetCode [227] Basic Calculator II

227. Basic Calculator II

Medium

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, /operators and empty spaces . The integer division should truncate toward zero.

Example 1:

Input: "3+2*2"
Output: 7

Example 2:

Input: " 3/2 "
Output: 1

Example 3:

Input: " 3+5 / 2 "
Output: 5

Note:

• You may assume that the given expression is always valid.
• Do not use the eval built-in library function.

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//C++: stack 36ms class Solution { public: int calculate(string s) { stack<int> nums; stack<char> ops; int lhs = 0, rhs = 0, n = s.size(); for(int i=0; i<n; ++i){ if(s[i]=='+'||s[i]=='-'||s[i]=='*'||s[i]=='/'){ ops.push(s[i]); }else if(s[i]!=' '){ rhs = rhs*10 + s[i] - '0'; if(i==n-1 || !isdigit(s[i+1])){ if(ops.empty() || ops.top()=='+' || ops.top()=='-'){ nums.push(rhs); }else{ while(!ops.empty()&&(ops.top()=='*'||ops.top()=='/')){ char op = ops.top(); ops.pop(); lhs = nums.top(); nums.pop(); rhs = (op=='*')?lhs*rhs:lhs/rhs; nums.push(rhs); } } rhs = 0; } } } lhs = 0; while(!ops.empty()){ rhs = nums.top(); nums.pop(); char op = ops.top(); ops.pop(); lhs = (op=='+')?lhs+rhs:lhs-rhs; } return lhs+nums.top(); } }; ]]></script> <script class="brush: js" type="syntaxhighlighter"><![CDATA[ //C++: 16ms class Solution { public: int calculate(string s) { s += "+"; int n = s.size(), i = 0, val = 0, prev = 0, cur = 0; char op = '+'; while(i<n){ if(isdigit(s[i])){ cur = cur*10+s[i++]-'0'; }else if(s[i]!=' '){ if(op=='+'){ val = val+cur; prev = cur; }else if(op=='-'){ val = val-cur; prev = -cur; }else if(op=='*'){ val = val-prev+prev*cur; prev = prev*cur; }else if(op=='/'){ val = val-prev+prev/cur; prev = prev/cur; } op = s[i++]; cur = 0; }else{ i++; } } return val; } }; ]]></script> <script class="brush: js" type="syntaxhighlighter"><![CDATA[ //C++: 16ms class Solution { public: int calculate(string s) { int res = 0, lhs = 0, rhs = 0, n = s.size(); char op = '+'; for(int i=0; i<n; ++i){ if(isdigit(s[i])){ rhs = rhs*10 + s[i] - '0'; if(i==n-1||!isdigit(s[i+1])){ switch(op){ case '+': lhs += rhs; break; case '-': lhs -= rhs; break; case '*': lhs *= rhs; break; case '/': lhs /= rhs; break; } rhs = 0; } }else if(s[i]!=' '){ if(s[i]=='+'||s[i]=='-'){ res += lhs; lhs = 0; } op = s[i]; } } return res + lhs; } }; class Solution { public: int calculate(string s) { int sz = s.size(); int i = 0; int lhs = 0; int prv = 0; char op = '+'; while(i<sz){ if(s[i]==' '){i++; continue;} int rhs = 0; while(i<sz && isdigit(s[i])){ rhs = rhs*10+(s[i]-'0'); i++; } switch(op){ case '+': lhs += rhs; prv = rhs; break; case '-': lhs -= rhs; prv = (-1)*rhs; break; case '*': lhs -= prv; lhs += prv*rhs; prv = prv*rhs; break; case '/': lhs -= prv; lhs += prv/rhs; prv = prv/rhs; break; default:break; } if(i<sz) op = s[i++]; } return lhs; } };

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class Solution { public int calculate(String s) { Stack<Integer> nums = new Stack<>(); Stack<Character> ops = new Stack<>(); int v = 0; boolean hasV = false; s += '+'; for(char c:s.toCharArray()){ if(c==' ') continue; else if(Character.isDigit(c)){ v = v*10 + c - '0'; hasV = true; }else{//'+', '-' if(hasV) nums.push(v); v = 0; hasV = false; while(!ops.empty() && precedence(ops.peek())>=precedence(c)) evaluate(nums, ops); ops.push(c); } } return nums.pop(); } int precedence(char op){ switch(op){ case '+' : return 0; case '-' : return 0; case '*' : return 1; case '/' : return 1; default: return -2; } } void evaluate(Stack<Integer> nums, Stack<Character> ops){ int r = nums.pop(); int l = nums.pop(); char op = ops.pop(); int result = 0; switch (op) { case '+' : result = l+r; break; case '-' : result = l-r; break; case '*' : result = l*r; break; case '/' : result = l/r; break; default: break; } nums.push(result); } }