LeetCodeProblem-Soluation/Largest-rectangle-in-histogram at main · PrinceNagarkoti/LeetCodeProblem-Soluation · GitHub

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//brute force approach
class Solution {
    public int largestRectangleArea(int[] heights) {
        int res = 0;
        int n = heights.length;
        for (int i = 0; i < n; i++) {
            int curr = heights[i];
            for (int j = i - 1; j >= 0; j--) {
                if (heights[j] >= heights[i])
                    curr += heights[i];
                else
                    break;
            }
            for (int j = i + 1; j < n; j++) {
                if (heights[j] >= heights[i])
                    curr += heights[i];
                else
                    break;
            }
            res = Math.max(res, curr);
        }
        return res;
    }
}

// optimzation Solution
class Solution {

    public int[] previousSmaller(int[] heights) {
        int n = heights.length;
        int[] ans = new int[n];
        Stack<Integer> stack = new Stack<>();

        for (int i = 0; i < n; i++) {
            while (!stack.isEmpty() && heights[stack.peek()] >= heights[i]) {
                stack.pop();
            }
            ans[i] = stack.isEmpty() ? -1 : stack.peek();
            stack.push(i);
        }
        return ans;
    }

    public int[] nextSmaller(int[] heights) {
        int n = heights.length;
        int[] ans = new int[n];
        Stack<Integer> stack = new Stack<>();

        for (int i = n - 1; i >= 0; i--) {
            while (!stack.isEmpty() && heights[stack.peek()] >= heights[i]) {
                stack.pop();
            }
            ans[i] = stack.isEmpty() ? n : stack.peek();
            stack.push(i);
        }
        return ans;
    }

    public int largestRectangleArea(int[] heights) {
        int n = heights.length;
        int res = 0;

        int[] ps = previousSmaller(heights);
        int[] ns = nextSmaller(heights);

        for (int i = 0; i < n; i++) {
            int curr = (ns[i] - ps[i] - 1) * heights[i];
            res = Math.max(res, curr);
        }
        return res;
    }
}