LeetCode-150. Evaluate Reverse Polish Notation (JAVA)逆波兰表达式求值

150. Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:

  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

从左到右遍历表达式的每个数字和字符，遇到数字就进栈，遇到符号，就将栈顶的两个数字取出（注意第一次取出的是右操作数，第二次取出的栈顶数字是左操作数），进行运算，将运算结果压栈，一直到最终获得计算结果（最终的栈顶数字）。

逆波兰式求值（后缀表达式）

public int evalRPN(String[] tokens) {int len;if ((len = tokens.length) == 0)return 0;Stack<Integer> stk = new Stack<>();for (int i = 0; i < len; i++) {try {int tmp = Integer.valueOf(tokens[i]);stk.push(tmp);} catch (Exception e) {// 栈的弹出顺序，先弹后面int sec = stk.pop();// 处理一个数字是负数的情况如{"3", "-" }// 或者 { "6", "3", "*", "-" }int fir = 0;if (!stk.isEmpty())fir = stk.pop();int tmp = 0;if (tokens[i].equals("+"))tmp = fir + sec;if (tokens[i].equals("-"))tmp = fir - sec;if (tokens[i].equals("*"))tmp = fir * sec;if (tokens[i].equals("/"))tmp = fir / sec;stk.push(tmp);}}return stk.pop();}