# 335. Self Crossing
You are given an array x of n positive numbers. You start at point (0,0) and moves x[0] metres to the north, then x[1] metres to the west, x[2] metres to the south, x[3] metres to the east and so on. In other words, after each move your direction changes counter-clockwise.
Write a one-pass algorithm with O(1) extra space to determine, if your path crosses itself, or not.
Example 1:
┌───┐
│ │
└───┼──>
│
Input: [2,1,1,2]
Output: true
Example 2:
┌──────┐
│ │
│
│
└────────────>
Input: [1,2,3,4]
Output: false
Example 3:
┌───┐
│ │
└───┼>
Input: [1,1,1,1]
Output: true
# Solution
Approach 1: Identify the rule.
# Code (Python)
Approach 1:
# Code (C++)
Approach 1:
class Solution {
public:
bool isSelfCrossing(vector<int>& x) {
if (x.size() < 4)
return false;
for (int i = 3; i < x.size(); ++i)
{
if ((x[i] >= x[i-2] && x[i-1] <= x[i-3]) ||
(i >= 4 && x[i] + x[i-4] >= x[i-2] && x[i-1] == x[i-3]) ||
(i >= 5 && x[i] + x[i-4] >= x[i-2] && x[i-2] >= x[i-4] &&
x[i-1] <= x[i-3] && x[i-1] >= x[i-3] - x[i-5]))
return true;
}
return false;
}
};