• I expect this problem to be medium problem.

Intuition

• Let chopper help explain.
• Starting at the origin and face north (0,1), after one sequence of instructions
  1. if chopper return to the origin, he is obvious in an circle.
  2. if chopper finishes with face not towards north, it will get back to the initial status in another one or three sequences.

Explanation

• (x,y) is the location of chopper.
• d[i] is the direction he is facing.
• i = (i + 1) % 4 will turn right
• i = (i + 3) % 4 will turn left
• Check the final status after instructions.

Complexity

Time O(N) Space O(1)

class Solution {
public:
    bool isRobotBounded(string instructions) {
        int x = 0, y = 0, i = 0;
        vector<vector<int>> d = {0, 1}, {1, 0}, {0, -1}, { -1, 0}; // 원래는 {} 하나 더 
        // facing : north, rigth, south, left
        for(char & ins : instructions)
        {
            if(ins =='R')
            {
                i = (i+1) % 4;
            }
            else if (ins =='L')
            {
                i = (i+3) % 4;
            }
            else
            {
                x += d[i][0], y += d[i][1];
            }
        }
        return x==0 && y==0 || i>0;

    }
};