<p>Given a binary array <code>nums</code>, return <em>the maximum length of a contiguous subarray with an equal number of </em><code>0</code><em> and </em><code>1</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> nums = [0,1]
<strong>Output:</strong> 2
<strong>Explanation:</strong> [0, 1] is the longest contiguous subarray with an equal number of 0 and 1.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> nums = [0,1,0]
<strong>Output:</strong> 2
<strong>Explanation:</strong> [0, 1] (or [1, 0]) is a longest contiguous subarray with equal number of 0 and 1.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
	<li><code>nums[i]</code> is either <code>0</code> or <code>1</code>.</li>
</ul>

from collections import defaultdict
class Solution:
    def findMaxLength(self, nums: List[int]) -> int:
        #0  0  1  1  0  0  0  0  0  1  1  1
       # 0 -1 -2 -1  0 -1 -2 -3 -4 -5 -4 -3 -2
        prefix_sum = [0]*(len(nums)+1)
        prefix_sum_dict = defaultdict(int)
        # if(nums[0]==0):
        #     prefix_sum[0] = -1
        # else:
        #     prefix_sum[0] = 1
        max_ans = 0
        count = 0
        prefix_sum_dict[0] = 0
        for i in range(len(nums)):
            if nums[i]==0:
                count-=1
                if count in prefix_sum_dict:
                    max_ans = max(max_ans, i+1-prefix_sum_dict[count])
                else:
                    prefix_sum_dict[count] = i+1
            elif nums[i]==1:
                count+=1
                if count in prefix_sum_dict:
                    max_ans = max(max_ans, i+1-prefix_sum_dict[count])
                else:
                    prefix_sum_dict[count] = i+1
        print(prefix_sum_dict)
        return max_ans
        
        

        
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