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`/**
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 * @param {number[][]} heights
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 * @return {number[][]}
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 */
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var pacificAtlantic = function(heights) {
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    let atlantic = new Set();
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    let pacific = new Set();
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    let rows = heights.length;
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    let cols = heights[0].length;
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    for (let c = 0; c < cols; c++) {
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        explore(heights, 0, c, pacific, heights[0][c]); // dfs from top row
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        explore(heights, rows - 1, c, atlantic, heights[rows - 1][c]); // dfs from bottom row
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    }
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    for (let r = 0; r < rows; r++) {
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        explore(heights, r, 0, pacific, heights[r][0]); // dfs from left most column
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        explore(heights, r, cols - 1, atlantic, heights[r][cols - 1]); // dfs from right most column
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    }
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    // check if water can flow to both atlantic and pacific ocean.
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    let res = [];
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    for (let r = 0; r < rows; r++) {
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        for (let c = 0; c < cols; c++) {
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            let pos = r + ',' + c;
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            if (atlantic.has(pos) && pacific.has(pos)) {
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                res.push([r, c]);
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            }
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        }
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    }
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    return res;
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};
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function explore(heights, r, c, visited, prevHeight) {
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    let rowInbound = 0 <= r && r < heights.length;
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    let colInbound = 0 <= c && c < heights[0].length;
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    if (!rowInbound || !colInbound) return;
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    // height must be higher than prev height. water can only flow downwards not upwards. duh.
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    // if it's the first value then it's just the same value so it's not less than so it will not return.
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    if (heights[r][c] < prevHeight) return;
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    let pos = r + ',' + c;
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    if (visited.has(pos)) return;
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    visited.add(pos)
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    explore(heights, r + 1, c, visited, heights[r][c]) // below
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    explore(heights, r - 1, c, visited, heights[r][c]) // above
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    explore(heights, r, c + 1, visited, heights[r][c]) // right
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    explore(heights, r, c - 1, visited, heights[r][c]) // left
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}`;