# 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;
    }
};