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class Solution {
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  public boolean validMountainArray(int[] arr) {
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    // edge case
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    if (arr.length < 3) return false;
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    // keep 2 pointers
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    int i = 0;
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    int j = arr.length - 1;
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    // use i pointer to iterate through steep increase from LHS
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    while (i < j && arr[i] < arr[i + 1]) {
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      i++;
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    }
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    // use j pointer to iterate steep increase from RHS
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    while (j > i && arr[j] < arr[j - 1]) {
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      j--;
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    }
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    // both should meet at same place and it be neither start or end.
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    return i == j && i < arr.length - 1 && j > 0;
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  }
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}