[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 = [2, 1, 1, 2],
Return true (self crossing)

Example 2:

Given x = [1, 2, 3, 4],
Return false (not self crossing)

Example 3:

Given x = [1, 1, 1, 1],
Return true (self crossing)

https://leetcode.com/problems/self-crossing/

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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;
};