[LeetCode][JavaScript]Self Crossing

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:

Given x = [code][2, 1, 1, 2]

,
Return true (self crossing)
[/code]

Example 2:

Given x = [code][1, 2, 3, 4]

,
Return false (not self crossing)
[/code]

Example 3:

Given x = [code][1, 1, 1, 1]

,
Return true (self crossing)[/code]
https://leetcode.com/problems/self-crossing/

逆时针画线，求线是否有交叉。

看图，三种情况，对应了代码中的注释。

/**
* @param {number[]} x
* @return {boolean}
*/
var isSelfCrossing = function(x) {
for(var i = 3; i < x.length; i++){
//i. [2, 1, 1, 2]
if(x[i - 3] && x[i] >= x[i - 2] && x[i - 1] <= x[i - 3])
return true;
//ii. [1, 1, 2, 1, 1]
if(x[i - 4] && x[i - 3] === x[i - 1] && x[i - 4] + x[i] >= x[i - 2])
return true;
//iii. [1, 1, 2, 2, 1, 1]
if(x[i - 5] && x[i - 4] <= x[i -2] && x[i] + x[i - 4] >= x[i -2]
&& x[i - 3] >= x[i - 1] && x[i - 5] + x[i - 1] >= x[ i - 3])
return true;
}
return false;
};