Basic Calculator II LeetCode solution in Java

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class Solution {
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  public int calculate(String s) {
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    if (s == null || s.length() == 0) return 0;
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    Stack<Integer> st = new Stack<>();
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    int curr = 0;
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    char op = '+';
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    char[] ch = s.toCharArray();
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    for (int i = 0; i < s.length(); i++) {
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      if (Character.isDigit(ch[i])) {
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        curr = curr * 10 + ch[i] - '0';
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      }
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      if (!Character.isDigit(ch[i]) && ch[i] != ' ' || i == ch.length - 1) {
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        if (op == '+') {
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          st.push(curr);
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        } else if (op == '-') {
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          st.push(-curr);
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        } else if (op == '*') {
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          st.push(st.pop() * curr);
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        } else if (op == '/') {
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          st.push(st.pop() / curr);
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        }
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        op = ch[i];
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        curr = 0;
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      }
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    }
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    int sum = 0;
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    while (!st.isEmpty()) {
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      sum += st.pop();
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    }
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    return sum;
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  }
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} // TC=o(n),SC=o(n)