The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.
You may assume that the given expression is always valid.
Some examples:
"3+2*2" = 7
" 3/2 " = 1
" 3+5 / 2 " = 5
Note: Do not use the eval built-in library function.
[分析]
思路1: 将输入先转换为逆波兰表达式,然后计算逆波兰表达式。该类型题目标准解法。
思路2:参考 https://leetcode.com/discuss/41902/share-my-java-solution
特别注意代码中的这个条件:if (!Character.isDigit(c) && c != ' ' || i == len - 1)
算法的思路是先解析一个新的运算数,然后判断其前面的运算符,若是乘除则立即算出结果放入栈中,若是正负号则与当前数字结合加入栈中,最后栈中数累加即为结果。
public class Solution { // Method 2 public int calculate(String s) { if (s == null || s.length() == 0) return 0; List<String> revPolish = reversePolish(s); return evalRPN(revPolish); } public List<String> reversePolish(String s) { List<String> tokens = new ArrayList<String>(); LinkedList<Character> signs = new LinkedList<Character>(); for (int i = 0; i < s.length(); i++) { int j = i; int num = 0; while (i < s.length() && '0' <= s.charAt(i) && s.charAt(i) <= '9') { num = num * 10 + s.charAt(i) - '0'; i++; } if (j < i) tokens.add(String.valueOf(num)); if (i == s.length()) break; char c = s.charAt(i); if (c == '+' || c == '-') { while (!signs.isEmpty()) { tokens.add(String.valueOf(signs.pop())); } signs.push(c); } else if (c == '*' || c == '/') { while (!signs.isEmpty() && (signs.peek() == '/' || signs.peek() == '*')) { tokens.add(String.valueOf(signs.pop())); } signs.push(c); } } while (!signs.isEmpty()) tokens.add(String.valueOf(signs.pop())); return tokens; } public int evalRPN(List<String> tokens) { LinkedList<Integer> nums = new LinkedList<Integer>(); for (int i = 0; i < tokens.size(); i++) { String curr = tokens.get(i); if (curr.equals("+")) { int num2 = nums.pop(), num1 = nums.pop(); nums.push(num1 + num2); } else if (curr.equals("-")) { int num2 = nums.pop(), num1 = nums.pop(); nums.push(num1 - num2); } else if (curr.equals("*")) { int num2 = nums.pop(), num1 = nums.pop(); nums.push(num1 * num2); } else if (curr.equals("/")) { int num2 = nums.pop(), num1 = nums.pop(); nums.push(num1 / num2); } else { nums.push(Integer.parseInt(curr)); } } return nums.pop(); } // Method 1 public int calculate1(String s) { if (s == null) return 0; LinkedList<Integer> stack = new LinkedList<Integer>(); char sign = '+'; int num = 0; int len = s.length(); for (int i = 0; i < len; i++) { char c = s.charAt(i); if (Character.isDigit(c)) num = num * 10 + c - '0'; if (!Character.isDigit(c) && c != ' ' || i == len - 1) { if (sign == '+') stack.push(num); else if (sign == '-') stack.push(-num); else if (sign == '*') stack.push(stack.pop() * num); else if (sign == '/') stack.push(stack.pop() / num); sign = c; num = 0; } } int res = 0; while (!stack.isEmpty()) res += stack.pop(); return res; } }