leetcode#777. Swap Adjacent in LR String

leetcode#777. Swap Adjacent in LR String

Problem Description:

In a string composed of ‘L’, ‘R’, and ‘X’ characters, like “RXXLRXRXL”, a move consists of either replacing one occurrence of “XL” with “LX”, or replacing one occurrence of “RX” with “XR”. Given the starting string start and the ending string end, return True if and only if there exists a sequence of moves to transform one string to the other.

Solution:
This problem is similar to quotes match more or less. The transformation can’t exchange the relative positions between L and R and X is useless. So we can solve the problem by stack easily. We scan the start and end simultaneously. Each time we meet an L in end or a R in start, we push it into the stack. On the other hand, each time we meet an L in start or a R in end we pop it from the stack. The condition that there exists a sequence of moves to transform one string to the other is that if and only if the stack is empty after the scanning.

Code:

class Solution {
public:
    bool canTransform(string start, string end) {
        stack<int> s;
        for (int i=0; iif (end[i] == 'L') s.push(1);
            if (start[i] == 'L' && !s.empty()) {
                if (s.top() == 1) s.pop();
                else return false;
            }
            if (start[i] == 'R') s.push(2);
            if (end[i] == 'R' && !s.empty()) {
                if (s.top() == 2) s.pop();
                else return false;
            }
        }
        return s.empty();
    }
};