Container With Most Water: the idea to remember.

The core insight

Evaluate the endpoints and move the shorter line inward.

Why it works

Keeping the shorter line while reducing width cannot improve the area. Discarding it cannot lose a better pair using that endpoint.

The mistake to avoid

Moving the taller endpoint has no justification for discarding candidates.

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

int maxArea(int[] height) {
    int l = 0, r = height.length - 1, best = 0;
    while (l < r) {
        best = Math.max(best, (r - l) * Math.min(height[l], height[r]));
        // The shorter line limits the area.
        if (height[l] <= height[r]) l++;
        else r--;
    }
    return best;
}

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